Starcraft Battle.net US Account (EU User)
Solution 1:
So here is the solution:
If you have an EU Account and a US CD-Key just go to us.battle.net and Sign In with your EU Account. Then add the CD-Key to your Account and download the enUS version of the game.
Solution 2:
I'm pretty sure you can go to us.battle.net and activate it. You may have to make a new account, though.