How do I insert data into MS Access file using SQL?

To start with, you need to put quotes around your text data:

@"INSERT INTO PersonalData (Type, UserName, Password) VALUES (" + cmbType.SelectedIndex + ",'" + txtUserName.Text + "','" + txtPassword.Text + "')";

However, you would be much better off converting this to use parameters, since you won't have to worry about embedded quotes:

objOleDbCommand.CommandText = @"INSERT INTO PersonalData (Type, UserName, Password) VALUES (?, ?, ?)";

objOleDbCommand.Parameters.Add("Type", cmbType.SelectedIndex); 
objOleDbCommand.Parameters.Add("UserName", txtUserName.Text);
objOleDbCommand.Parameters.Add("Password", txtPassword.Text);

Let's say txtUserName.Text is Foo, and txtPassword.Text is bar. Then you're getting

INSERT INTO PersonalData (Type, UserName, Password) VALUES (3,foo,bar)

instead of the syntactically correct

INSERT INTO PersonalData (Type, UserName, Password) VALUES (3,'foo','bar')

Since you don't have columns foo and bar, you are getting an error - is my assumption. Things get worse if you have baz,moo instead of foo. Or, gods forbid, Bobby Tables.

There a few places where you might encounter problems here.

1. Type, Username, and Password are all (I think) MS Access keywords. While they seem to work when used within Access istself (like in the Querybuilder, for example), they seem to throw exceptions when used from Client Code. Surround the fieldnames in your SQL Statement with square brackets, so that Access treats them as literals.

2. I strongly recommend using SQL Parameters for your in-line SQL, and then using ADO.NET Parameters to set the values. Google "SQL Injection Attack" to learn why. Plus, it's just good practive (there are some limited exceptions).

EDIT: Note that with OleDb, the parameters must appear in the same order as the fliednames in the list. THis is not the case with ADO & SQLClient. With Access, however, having your parameters out of order will create difficult-to-find problems . . .

Your SQL would then look like this:

INSERT INTO ([Type], [Username], [Password]) VALUES ( @Type, @UserName, @Password )

And your code might resemble THIS (I took some liberties here . . .

private void InsertUserData(int Type, String UserName, String Password)
{
    // The "Using" block handles object creation and disposal - 
    // handy for unmanaged resources like database connections:
    using(OleDbConnection cn = new OleDbConnection(YourConnectionString))
    {
        using(OleDbCommand cmd = new OleDbCommand())
        {
            cmd.Connection = cn;

            // 1.  Note the use of Parameters here. This will hinder attempts to 
            // compromise your app with SQl Injection and/or faulty user input. 

            // 2. Also note that, since "Type", "Username", and "Password" are all 
            // MS Access keywords, there is a potential for problems when 
            // used as fieldnames. Therefore we enclose them 
            // in square brackets [] in the "INSERT INTO" Clause:
            String SQL =
                "INSERT INTO PersonalData([Type], [UserName], [Password]) " +
                "VALUES(@Type, @UserName, @Password)";

            // Set the CommandText Proprty:
            cmd.CommandText = SQL;

            // Now create some OleDb Parameters:
            OleDbParameter prmType = new OleDbParameter("@Type", Type);
            OleDbParameter prmUserName = new OleDbParameter("@UserName", UserName);
            OleDbParameter prmPassword = new OleDbParameter("@Password", Password);

            // Add the params to the parameters collection:
            cmd.Parameters.Add(prmType);
            cmd.Parameters.Add(prmUserName);
            cmd.Parameters.Add(prmPassword);

            try
            {
                cn.Open();
                cmd.ExecuteNonQuery();
                cn.Close();
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }
}

Hope that helps . . .