First time using ArcGIS JavaScript API - where do I find the JavaScript API to include (.js) into my solution?

Javascript location [Current as 13/05/2013]

http://serverapi.arcgisonline.com/jsapi/arcgis/3.5/

For the proxy settings

note the code:

esri.config.defaults.io.alwaysUseProxy = true;

<script type="text/javascript">
    dojo.require("esri.map");
    dojo.require("dijit.layout.ContentPane");
    dojo.require("dijit.layout.BorderContainer");
    var map;
    function Init() {

        esri.config.defaults.io.alwaysUseProxy = true;
        esri.config.defaults.io.proxyUrl = "http://WebServerName/proxy.ashx"; 
        dojo.style(dojo.byId("map"), { width: dojo.contentBox("map").w + "px", height: (esri.documentBox.h - dojo.contentBox("navTable").h - 40) + "px" });
        map = new esri.Map("map");

        var CountyDataLayer = new esri.layers.ArcGISDynamicMapServiceLayer("http://Server.Name/ArcGIS/rest/services/Counties/MapServer");
        map.addLayer(CountyDataLayer);

        var RestrictedLayer = new esri.layers.ArcGISDynamicMapServiceLayer("http://Server.Name/ArcGIS/rest/services/cadaster/ServiceName/MapServer");
        map.addLayer(RestrictedLayer);


        var resizeTimer;
        dojo.connect(map, 'onLoad', function(theMap) {
            dojo.connect(dijit.byId('map'), 'resize', function() {
                clearTimeout(resizeTimer);
                resizeTimer = setTimeout(function() {
                    map.resize();
                    map.reposition();
                }, 500);
            });
        });
    }
    dojo.addOnLoad(Init);
</script>

... and I used the following code in my proxy.config file:

Code:

<?xml version="1.0" encoding="utf-8" ?>
<!-- Proxy config is used to set the ArcGIS Server services that the proxy will forward to.        
        mustMatch: true to only proxy to sites listed, false to proxy to any site -->
<ProxyConfig mustMatch="true">
    <serverItems>

        <serverItem url="http://Server.Name"
                    matchAll="true" tokenUrl="http://Server.Name/ArcGIS/tokens"
                    username="MyUserName" password="MyPassword"
                    timeout="5" />

    </serverItems>
</ProxyConfig>

this code is taken from

http://forums.arcgis.com/threads/36823-How-do-you-reference-the-proxy.ashx-page