At the very least, let this be a warning to all others...
<devils advocate>
Even I'm not a marketing or
SEO expert, but as soon as I went to the site, thanks to Google's toolbar, I saw that it had a
PR of 0...that, combined with an alexa rank of 3M+, 5 sites linking in, etc....well....
Indeed, do a little more research next time instead of seeing $ signs and jumping in head first. Did you even ask for proof of their track record with other advertising campaigns that would show that they were capable of providing 40+ signups per month?
http://www.alexa.com/siteinfo/p4ks.com
</devils advocate>
Even so, I hope you get your money back, but I seriously doubt that you will.