How can I activate my iTunes gift card?

Take it back to the store you bought it from and ask them to activate it properly. If they can't or won't do so try contacting iTunes Support (they are likely to want to see images of the front and back of the card and its receipt) : http://www.apple.com/support/itunes/ww  (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)