Yeah basically you can only use steam if you are using the key that u had when u bought arma 2 OA. If you changed your key to any random key, you will either need the steam acount with the key, or download "DayZ Commander" which is basically a launcher for dayz that allows u to easily download/update any arma 2 or dayz maps. Good Luck!
It works. Though i think i should point this out. Change the key, then do it. Then if your key gets banned undo the deny change key then redo it. The reason for this is because after you deny the set value the key changer stops working so you must undo the changes and redo them. Sorry its hard to explain.