I can't get my iTunes voucher to activate

If the store that you bought it from didn't activate it properly then is it nearby so that you could try going back and asking them to do so ? If the store isn't close, or they can't/won't activate it, then 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