Multiplayer error

I´ve just bougt the game, one week ago. No problems with the singelplayer, and now i thougt that i want to try out muliplayer version. to try out batteling agenst other people.

But it dosent work... When im toughing the buttom multiplayer it says=''The key not found''..
im trying to retype the key again and again, but nothing hapends..??
Then, how do i activate my account, so i can play online.

Hope to hear