I think you need to open up ports 4443 and 5190. One is for direct connection, the other for file xfer, I think. Go into the router's config menu and go to Advanced, then Forwarding, and add entries for those ports to forward to your PC.

Also check this out, apparently AIM made some software changes to make it easier to work from behind firewalls. Haven't used it, since I use Trillian, but it might help you out.