I have recently bought a few Cards, and here is what you do..
The Card/code is good for ALL zones/regions of the game.
1: Scratch the card
2: Go to www . worldofwarcraft . com/misc/en/promotion.html
3: Enter your region, server, card-code and CAPTCHA
4: You get a new code, which you can ONLY use on the server you specified above.
5: Fly to Booty Bay, pay Landro a visit and give him the code you got from website.
Sorry, post-acount too low to make real URLs