I don't believe you can. Afaik, the servers are independent of one another so the only real option you have is to return the game to where you got it from and try to get a US version. Or you could contact Turbne directly and let them know what happened, they may be able to help you but I doubt it.
Alternatively, if you have not used the code in it yet, you could always buy a US game then sell the one you have, just make sure you include the buddy key since it came with it.