Well, when you start play WoW the first time, and is going to make your account, you HAVE to pick payment method. BUT, if you first make an account with your guest pass 10-day key, you can upgrade your account to full with the real key. So:
1. Start making an account with your guest-pass key.
2. When they say Payment method, press Visa(NOT electron), and type in one of the following numbers:
4532523177760998
4556140223976030
4024007132263947
4539559590631864
When you are to type Expiration Date; Pick anything, but it has to be a date in the future..
The security key is the 3 last numbers of the credit card number, so for 4532523177760998, it is 998, for 4556140223976030, it is 030 and so on.
3. Fill in the rest of the info and complete the account creation.
4. Go to account management, and press Upgrade Account.
5. Type in the CD-key of your full-version WoW and press continue.
6. Go to "Change Subscription" and then "Cancel Subscribtion".
7. Grats! You now got 1 month and 10 days on your playing time.