The prefix is used for if you want something to appear before the facebook status in your PSM, so for example you might want:
"
Facebook Status: Danny is happy"
the bit in italics would be the prefix. It's an optional thing, if you just leave it blank it will just display your status as the personal message.
You find the feed link by clicking on the link in the options window, then scrolling to where you can see 'My Status', then right click on that and choose 'copy' then right click in the feed link box in the options window and choose paste.
That's all explained in the first post