This is one of the best resources around: http://portforward.com/
1. The client does nothing.
2. The host finds out their local behind-their-firewall IP address.
3. The host logs into their router.
4. The host forwards the single AI War port (configurable in settings if you wish) to their behind-their-firewall local IP.
5. The host finds their external IP (http://www.whatismyip.com/
) and gives that to the client.
6. The client connects to the host via the external IP, using the same port that was set up in #4.
7. If there are still problems, make sure that there are not any software firewalls on the client or server side that are interfering (sometimes antivirus software includes firewalls you might not even be aware of). There needs to be an exception in any and all software firewalls on the client and server side for AI War and its port.
Hope that helps!