Instructions
Scratch off the gray code covering on the back of the Xbox Live Content card, Xbox Live Gold Membership card or Xbox Live/Microsoft Points card to reveal the 25-character prepaid code.
Push the Guide button, found between the Start and Select buttons on your Xbox 360 controller, to bring up the Guide window.
Choose the "Sign In" option. You will be prompted to choose a gamer profile. Select the profile you would like to use with the Xbox Live card.
Scroll over to the "Marketplace" tab.
Choose the "Redeem Code" option.
Type in the 25-character prepaid code, and push the Start button.
Select the "Confirm Code" option. Depending on the Xbox Live card, you will either receive Xbox Live Points, Xbox Live Gold Membership or an exclusive content item.