Redeeming a World of Warcraft Pre-paid Card is really easy, just follow these steps:
1/ Log in to your Battle.net Account Management.
2/ Pick the World of Warcraft license you want to manage.
3/ Select Game Time & Subscriptions.
4/ Choose Redeem a Prepaid Card and click Continue.
5/ Put the prepaid game-time code in and click Claim Code.
That's it, your subscription has just been prolonged!
The key is not working
If you have activated the key according to the guide but the code appears not to work, please check what type of error message you receive and depending on it follow the appropriate instructions: