Why Doesn't My FaceTime Work?

FaceTime works on my iPod Touch, but not my Mac. Doesn't matter which Apple ID I use, I always get the same error. "FaceTime cannot connect. Check your network settings and try again." Or something like that. I can log into the iTunes Store with the same Apple ID, and my Safari works fine. I've tried with the firewall On and Off. I've tried deleting the App and reinstalling from the App Store. No success.

Thanks!