Click on the Profile link for the forums:
http://www.sweepsadvantage.com/smf/index.php?action=profile
Click on Forum Profile Information under Modify Profile
Scroll down until you see Signature:
add
<URL=http://prizewish.prodege.com/?cmd=sb-address&new=1&rb=28146><img>
http://www.sweepsadvantage.com/adpeeps/uploaded_banners/137x63PW.gif</img></URL>
where 28146 is you change to your id and change <> to square brackets. [ ]
You need to get this from Prizewish. Only way I saw how to get this was to click on Win with Your Friends (
http://prizewish.prodege.com/?cmd=sb-refer) and send email to yourself.