By default, iChat tries to connect to the AIM network for AIM/.Mac/MobileMe accounts using port 5190:
If a firewall you don't control blocks port 5190, then click the SSL checkbox:
Chances are, you can now logon to the AIM network using your AIM/.Mac/MobileMe account through port 443, the SSL port. Your network overlords would have to be blocking access to specific AIM hostnames for this not to work. While entirely possible, far less likely than port blocking. No guarantee that video iChat will work though...