hope so ,but still got problem whit kickinout from servers
seriously, if you got the key from steam after you bought a valid copy there, it has to work. Otherwise you have to contact the service and provide proof that it doesnt work.
But I am pretty sure thats not the case and you just simply missed something.
This might help you: right click M&B warband on your library, chose "view CD key", you probably recognize if this is the one you just bought or probably another one from a cracked version.