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You can only use the key with one account, so if you already typed in the key with that account don't bother to create another because it will say that the key is in use.
As for not being able to log in with that account (the one you used from Mirror's Edge), check if you can login from a browser first (https://profile.ea.com/login.do ). If your browser logs you in well but the game doesn't then there must be some firewall problem, or a port closed.