You can redeem your Microsoft Currency by following these steps:
- Sign into Xbox LIVE using the account for which you want to use the Microsoft Currency/ Prepaid Card.
- Press the Guide button on the controller (big "X" button in the middle).
- Go to Games & Apps and select Redeem Code.
- Enter the 25-character code and select "Done"
Once you have done so, this currency will be added to your Microsoft Live Account!