You need to use stations that broadcast in mp3, http://www.shoutcast.com is a good place to start.
Right click on the "TUNE IN" button on a shoutcast mp3 station then click on "Save link/target as..."
Save the .pls file on your computer (desktop?!) then open that file in a text editor, (like notepad).
Here is the file .pls from BBC Radio1
Now you can just use the station's http:// address directly in the Argon xmb.xml you don't need to point to the .pls file, just add another station to the "favorites" section of the xmb.xml:
Title1=(#1 - 16/100) BBC Radio1
<station>BBC Radio 1</station>