Right here's what I sent him.
This is just for the DMZ, nothing to do with WEP.
"Here’s what you need to do to make it so you don’t have problems joining games etc.
Start up the 360, go to the Network blade, go to edit settings, go to additional settings and then advanced settings. Take a note of the letters and numbers under the “Wired Mac Address”
Open up Firefox on the PC and type in this:-
http://www.routerlogin.net
Login with the username of admin and password of password
Click on the LAN IP Setup
Click on the Add button
In the IP Address box, type in 192.168.1.20
Type the MAC Address of the Xbox 360 that you got above
Click Apply to enter the reserved address into the table.
Click on the WAN Setup
Look for the default DMZ server setting, change the last digit to 20 and then put a tick in the box, click on Apply.
Reboot the router and the Xbox 360 and it should now be all setup so you can access everyone’s games"