Instructions
Purchase a Limited Edition copy of "Halo 3." Depending on where you purchase it, you might still find new copies with codes guaranteed to work. By purchasing used copies, you might not get a code that works.
Remove the code from the case. This will be tucked on top of the manual inside the game case.
Redeem the code. Press the "Guide" button on your Xbox 360 controller. Navigate to "Marketplace" and then "Redeem Code." Press "A." Type in the code and press "Done."