I think the problem lays within your LIVE ID.
--> there you will find your XBOX Live account, which was probably created when activating the phone with your ID. My guess is that the country (as it is with my account too) is set to United States.
This way you cannot register your creditcard, as it's not valid for the united states.
The next problem: The country cannot be changed at the moment. There are severeal threads at the Microsoft help center, adressing this issue. However they, despite being aware of the problem, did not come up with a solution.
The only solution is creating a new account and insertig correct Data (pay attention to the location set on your computer and phone, because Zune software automatically detects your location from there).
Let me know if that helped.