Taken from our sticky here:

http://eu.battle.net/wow/en/forum/topic/1172960406

US Box Versions

Please be aware that US CD-Keys are unfortunately not compatible with EU region accounts, so if you appear to have accidentally acquired a US box version of Cataclysm then you will need to contact our Account and Payment department directly, making use of the email address WoWBillingSupportEU@Blizzard.com and including the following information in your mail;

o A clear and readable digital image of your US Authentication Key
(taken from the game box itself)
o Your Full Name
o Street Address
o Town or City
o Post Code
o Country
o Phone Number



As well as your Battle.net account name, and any other information you think may be of importance.

Good luck getting this sorted Mihirin. :)

Good afternoon Mihirin,

I believe I can see the email you’re referring to, but I’m afraid that as it contained roughly the same request as your first email, and we’re unable to accept the information you provided (for reasons I’ll go into in just a moment), you unfortunately may not actually receive a further response to that particular submission.

The issue with your second mail, I believe, is that the image of your US CD-Key needs to be attached to your webform using the available means of uploading images, as we’re regrettably unable to accept URL links or images uploaded onto third party sites.

If possible, please can you resend the information and attach the necessary image in the way I mention, and if you post again once you’ve done this then I’ll be able to check and confirm whether the mail has arrived and that it contains everything we need. :)

The email looks good, and I could access all the attachments without issue, so fingers crossed that we’ll be able to sort this out for you once your mail reaches the top of the queue. :)

Do let us know if you have any other issues, or don’t receive a response within a reasonable number of days (I myself will be back in on Friday after tonight, but we’ve got pandas in all week), and if necessary we’ll chase things up for you!