I don't have AIM here at work (to tell you EXACTLY how to do this), but I know a quick way to resolve this issue.

Since "the internet" (web) works, then it sounds like you can make outbound connections on "port 80."

There is a 'Connection Preferences' section in AIM and I believe the default port is 5180. Go ahead and CHANGE the port from 5180 to 80 and click OK.

This should resolve your issue. If you still can't find the connection peferences area, let me know and I can let you know the exact steps to take when I get home.