First you need to find your registered computers MAC address.

Windows XP

1. Click Start, then Run, then type "cmd" in the text box and hit enter.
2. Type in "ipconfig /all" in the Command Prompt Window.
3. Scroll that list and look for Ethernet adapter Local Area Connection.
4. The 12-digit Physical Address is the same as MAC address
5. Copy down the address it should look like this: 00-06-5B-15-04-B

MAC OS X

1. Open System Preferences (usually found in the Dock) and select Network.
2. Pick a location and specify the interface whose MAC address is desired in the Show: drop down list
3. For Built-in Ethernet, select the Ethernet tab.
4. Copy the MAC address labeled Ethernet ID, it should look like this: 00:0a:95:d1:52:30

Second you need to enter the MAC address, you copied, into the Xbox360.

1. Turn on your Xbox 360 without a disc in the disc tray.
2. In the System section of the Xbox Dashboard, select Network Settings.
3. Select Edit Settings, and then select the Additional Settings tab.
4. Select Advanced Settings Information.
5. Select Alternate MAC address, and then use the on-screen keyboard to enter the MAC address you copied from your computer.
6. Select Done.
7. Now plug your Xbox360 into the Ethernet wall jack.
8. Select Apply and select Test when you are prompted.

From the Xbox Dashboard, choose Xbox Live. Or, play a game on your Xbox Live Starter Kit disc and choose the option for Xbox Live play.

If you see the Xbox Live screen, you are connected to Xbox Live over Purdue ResNet.

If your Xbox console can't connect to Xbox Live, the Xbox Dashboard provides an automatic network troubleshooter to help you connect.

Microsoft (XBox) page with connection instructions:
http://www.xbox.com/connect/

Please refer to your game console's documentation or the manufacturer help line for any further assistance.

Remember you must register your computer with ResNet before trying to connect your Xbox360.