First, find out the IP address of the Xbox 360.
Then log in to the web interface of the router, click on Advanced, then Network Settings->Address Translation. At the bottom, you'll see the NAT table. Delete any entries in that table before continuing.
Click Enter NAT Table. Select New Group, and name the group XBL for Xbox Live. Then use the following settings:
IP Address of WAN: leave on Airstation's WAN IP address.
Protocol (WAN): TCP/UDP. select either TCP/UDP, and enter the port number.
IP Address of LAN: set to the IP address of the xbox 360
Protocol (LAN): leave this field blank.
Create a new nat table entry for each port: UDP 88, TCP 3074, and UDP 3074.