iTunes Store: How to redeem an iTunes Gift Card – If you have an iTunes Gift Card that you want to redeem in the iTunes Store, follow the appropriate tutorial below.  If you want to learn how to redeem an iTunes Store online Gift Certificate, refer to iTunes Store: How to use your iTunes gift certificate.
Note: Some features or services are not available in some iTunes Store countries.
 

http://support.apple.com/kb/HT1574