Instructions
Go to the "Registration" portion of Xbox.com. See References for the direct link.
Click the "Sign in" link to direct you to a Windows LIVE ID sign-in page.
Log into your Windows LIVE ID by entering your email and password into the corresponding text fields. You will be directed to the "Device Service" home page.
Choose "Register a new device" from the "Other Options" text menu list.
Enter all of your console and personal information into the fields on the resulting registration page.
Click the green "Submit" button to submit your registration information to Xbox.com and finish registering your console.