I was struggling to find an XJSON data source example (That is, a data source that is capable of loading JSON from a domain other than where the scripts resides), so battled through it myself.
The example below shows how to do it.

// Create your XJSONDataSource object. Its just a standard data source with a few whistles.

DataSource ds = new XJSONDataSource();
ds.setRecordXPath("/item");

// Set it up with whatever fields you want, as per usual

DataSourceTextField name = new DataSourceTextField("name", "Data Name");
name.setValueXPath("name");

DataSourceIntegerField id = new DataSourceIntegerField("id", "Data Id");
id.setValueXPath("id");
id.setPrimaryKey(true);

ds.setFields(id, name);

// DataSource is now ready for data. Give it a url.

ds.setDataURL("http://your_json_url_feed");

Once this loads (and remember to fetch the data through your tree grid or whatever else is viewing it), it will send a request that looks something like this:

http://your_json_url_feed?callback=isc.Comm._scriptIncludeReply_0

The JSON that your server must send must be wrapped around this callback label. For example:

isc.Comm._scriptIncludeReply_0 ({"item": [ {"id": "1","name": "Monkey"},
{"id": "2","name": "Tree"},
{"id": "3","name": "Banana"} ] })

The callback value can change, so make sure your server code picks it up and puts it in the right place within the response accordingly.