If you receive a promo code it is simple to redeem. Just go to the home screen of the App Store and click "Redeem" in the upper  right hand corner. Enter your redemption code and sync your iPhone or iPod Touch. Make sure to do so immediately as promo codes do expire and cannot be replaced if this occurs. (Sorry but it only works in the US App Store. Also, please remember to provide feedback here and on iTunes after using your new app for a bit. Thanks.