First, you'd need to know what port Xbox Live uses. I have no idea sorry, as I don't have one. Then you'd want to add a 'Virtual Server'. Enter the IP address of your Xbox, the port that it needs to connect to in both 'Public Port Start' and 'Mapped Private Port'. You can leave the Public Port End empty. It will also want to know what protocol to use. It will most likely be TCP, although if you Google the ports, the info should also specify that. You may need to have 2 ports setup, one for TCP, one for UDP.
If you get stuck, just let us know and I'll try help :)
Failing that, you could get a router that supports UPnP! That pretty much allows software to configure the ports that it needs.