Personally, I have gotten real clicks and real conversions from their traffic but the typical day by day ROI is pretty bad with them. They recently got bought by exact advertising (exactadvertising.com). Give them a call and see what's up. Tell them you want to spend a bunch of money with them but can't with the current state of traffic, let them squirm and see how they react.
I am advertising 2 different sites with them. The performance can be decent on goclick but you have to watch it closely. Out of all the PPCs no sites perform nearly as well as google adwords and overture.
You may also want to look into findwhat.com and lycos' ppc, we have had moderate success with those.
You used the term Addon domain, which makes me think your hosting control panel is cPanel. If this is the case, you should be able to check out your stats for that domain under "subdomain stats". You should see something like added-domain-name.primary-acount-domain-name.com
Just a shot in the dark, really.. If that doesn't work, e-mail your tech support, or look for the raw log file in your ftp account. Your host hopefully has some sort of way for you to see your stats.