Xbox 360s must be manually registered. This is done by finding the MAC address of the console and then using a computer with Internet access to visit the Add/Remove Registrations page.
Once you have logged into your MyUNIverse and accepted the ResNet use policy, enter your MAC address. Under the "description" section, be sure to put what kind of game console you are registering.
Below are the instructions for finding your XBox 360's MAC address.
(Menus since December 2011)
1. Press the Guide button on your controller.
2. Go to Settings, and then select System Settings.
3. Select Network Settings.
4. Choose Wired Network.
5. Select Configure Network.
6. Go to the Additional Settings tab, and then select Advanced Settings
7. On the advanced settings page, you will see Wired MAC Address listed. This is your MAC address.