Arma 2 OA installation Key Error

[Mankiw]

I whent to gamestop bought a Arma 2 OA CD KEY and Whent Home to test it out, when i enterd ADD A GAME on steam it just came your CD key is invalid. Then i tried to fix it manually it funktioned so i didnt under stand, Then when i took out the cd i SAID, you need the cd to play so ive been trying to fix it for my steam and WRONG code WRONG code / how to fix it

[[ESP] Titof92 [ADM]]

http://gyazo.com/fabe57507f3d9fbeb2bdfaac102187b4