What's the actual error message that you are getting, that the card hasn't been activated or that the code is invalid ?
If invalid code (and your iTunes account has a US billing address on your account ; all gift cards are country-specific) and this page doesn't help then you will need to try contacting iTunes Support (you will probably need to give them images of the front and back of the card, and possibly its receipt) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes
You can also use that link to contact Support if the card hasn't been activated.