Tenshinhan

Hi, i have a original KEY that i received from a friend that bought the game and gave me one copy. But i formatted my pc sometime ago and i don't have the game anymore (i still have the key) then i downloaded the game but when i install it it doesn't ask for the key(it is called Arma 2 Standalone Expansion pack) How can i insert the key? Yes, the key is legit! Thanks.