Access, parse, and write .csv data - google apps script

there were a few errors in your code. Here is a working version with comments on the lines I modified.

function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = 
      [ {name:"Read Data", functionName:"readRows"},
        {name:"Upload DSR", functionName:"doGet"} ];

  sheet.addMenu("Du-par's", entries);
};

//Adds a custom menu to the active spreadsheet, containing a single menu item
//for invoking the readRows() function specified above.

// Create Menu to Locate .CSV
function doGet(e) {

  var app = UiApp.createApplication().setTitle("Upload DSR");
  var formContent = app.createVerticalPanel();
  formContent.add(app.createFileUpload().setName('thefile'));
  formContent.add(app.createSubmitButton("Submit DSR"));
  var form = app.createFormPanel();
  form.add(formContent);
  app.add(form);
  SpreadsheetApp.getActive().show(app);
  return app;
}

// Upload .CSV to Cloud
function doPost(e) {
  var app = UiApp.getActiveApplication();  
  // data returned is a blob for FileUpload widget
  var fileBlob = e.parameter.thefile;
  var doc = DocsList.createFile(fileBlob); 
  // Display a confirmation message
  var label = app.createLabel('file uploaded successfully');
  app.add(label);  
  var files = DocsList.getFileById(doc.getId());// to get the document's data back you need to use the right method... fileBlob was not the right argument
  var csvFile = ""; // this part was not inside the doPost function, it wasn't included in any function thus trying to run each time any function is called, includind onOpen()... beware the {}...
  csvFile = files.getContentAsString();
  var csvData = CSVToArray(csvFile, ",");
  Logger.log(csvData)
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getActiveSheet();
  for (var i = 0; i < csvData.length; i++) {
    sheet.getRange(i+1, 1, 1, csvData[i].length).setValues(new Array(csvData[i]));
  }
  return app;// you don't need to re-call the show method, just return the active app to update the UI
}


function readRows() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var rows = sheet.getDataRange();
  var numRows = rows.getNumRows();
  var values = rows.getValues();

  for (var i = 0; i <= numRows - 1; i++) {
    var row = values[i];
    Logger.log(row);
  }
};



function CSVToArray( strData, strDelimiter ){
Logger.log(strData);
  // Check to see if the delimiter is defined. If not,
  // then default to comma.
  strDelimiter = (strDelimiter || ",");

  // Create a regular expression to parse the CSV values.
  var objPattern = new RegExp(
    (
      // Delimiters.
      "(\\" + strDelimiter + "|\\r?\\n|\\r|^)" +

      // Quoted fields.
      "(?:\"([^\"]*(?:\"\"[^\"]*)*)\"|" +

      // Standard fields.
      "([^\"\\" + strDelimiter + "\\r\\n]*))"
    ),
    "gi"
  );


  // Create an array to hold our data. Give the array
  // a default empty first row.
  var arrData = [[]];

  // Create an array to hold our individual pattern
  // matching groups.
  var arrMatches = null;


  // Keep looping over the regular expression matches
  // until we can no longer find a match.
  while (arrMatches = objPattern.exec( strData )){

    // Get the delimiter that was found.
    var strMatchedDelimiter = arrMatches[ 1 ];

    // Check to see if the given delimiter has a length
    // (is not the start of string) and if it matches
    // field delimiter. If id does not, then we know
    // that this delimiter is a row delimiter.
    if (
      strMatchedDelimiter.length &&
      (strMatchedDelimiter != strDelimiter)
    ){

      // Since we have reached a new row of data,
      // add an empty row to our data array.
      arrData.push( [] );

    }


    // Now that we have our delimiter out of the way,
    // let's check to see which kind of value we
    // captured (quoted or unquoted).
    if (arrMatches[ 2 ]){

      // We found a quoted value. When we capture
      // this value, unescape any double quotes.
      var strMatchedValue = arrMatches[ 2 ].replace(
        new RegExp( "\"\"", "g" ),
        "\""
      );

    } else {

      // We found a non-quoted value.
      var strMatchedValue = arrMatches[ 3 ];

    }


    // Now that we have our value string, let's add
    // it to the data array.
    arrData[ arrData.length - 1 ].push( strMatchedValue );
  }

  // Return the parsed data.
  return( arrData );

  // Using active sheet here, but you can pull up a sheet in several other ways as well
  //SpreadsheetApp.getActiveSheet()
    //            .getRange( 1, 1, values.length, values[0].length )
      //          .setValues(values);
} 

at the end of the doPost function you could use return app.close(); to automatically close the UI, no need to show a label with "upload completed" , the sheet shows the data so we know the upload process went right...

EDIT :

And here is a "compact" version of your code without the numerous comments and with a few improvements, notice the way I write the array data back to the spreadsheet using setValues() without loop. The UI is also slightly modified.

I saw in your other post that you wanted to delete the uploaded file from your drive so I skipped this step by getting the string content directly from the blob (see comment in code)

function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = 
      [ {name:"Read Data", functionName:"readRows"},
        {name:"Upload DSR", functionName:"doGet"} ];

  sheet.addMenu("Du-par's", entries);
};

function doGet(e) {  
  var app = UiApp.createApplication().setTitle("Upload DSR").setHeight(100).setWidth(300);
  var formContent = app.createGrid(3,1);
  formContent.setWidget(0,0,app.createFileUpload().setName('thefile'));
  formContent.setWidget(2,0,app.createSubmitButton("Submit DSR"));
  var form = app.createFormPanel();
  form.add(formContent);
  app.add(form);
  SpreadsheetApp.getActive().show(app);
  return app;
}

function doPost(e) {
  var app = UiApp.getActiveApplication();  
  var fileBlob = e.parameter.thefile;
  var content = fileBlob.getDataAsString();// no need to create an intermediate file
  var csvData = CSVToArray(content, ",");
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getActiveSheet();
  sheet.getRange(1,1,csvData.length,csvData[0].length).setValues(csvData);
  return app.close();
}

function readRows() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var rows = sheet.getDataRange();
  var numRows = rows.getNumRows();
  var values = rows.getValues();

  for (var i = 0; i <= numRows - 1; i++) {
    var row = values[i];
    Logger.log(row);
  }
};


function CSVToArray( strData, strDelimiter ){
  Logger.log(strData);
  strDelimiter = (strDelimiter || ",");
  var objPattern = new RegExp(
    (
      "(\\" + strDelimiter + "|\\r?\\n|\\r|^)" +
      "(?:\"([^\"]*(?:\"\"[^\"]*)*)\"|" +
      "([^\"\\" + strDelimiter + "\\r\\n]*))"
    ),
    "gi"
  );
  var arrData = [[]];
  var arrMatches = null;
  while (arrMatches = objPattern.exec( strData )){
    var strMatchedDelimiter = arrMatches[ 1 ];
    if (
      strMatchedDelimiter.length &&
      (strMatchedDelimiter != strDelimiter)
    ){
      arrData.push( [] );
    }
    if (arrMatches[ 2 ]){
      var strMatchedValue = arrMatches[ 2 ].replace(
        new RegExp( "\"\"", "g" ),
        "\""
      );
    } else {
      var strMatchedValue = arrMatches[ 3 ];
    }
    arrData[ arrData.length - 1 ].push( strMatchedValue );
  }
  return( arrData );
}