i need help

All call of duty games after MW1 were Steamworks.

authentication cd key is not a steam key!

Is it a normal retail box of the game? And if so, was the package still properly sealed in shrink-wrapped plastic, or something else that you can't remove, then put back on? Or was it only closed by a round little plastic sticker? Maybe the store sold you a copy they'd already given someone else, who bought it, then returned it, after having used the key already.

In any case, ignore people here telling you to contact Steam. They won't help you. They will just tell you what I am also going to tell you: this is the problem of the store who sold it to you, and you should talk to them.

But before you do - are you sure you're entering the key properly? Show me what the key looks like, its' format. No need (obviously) to post the actual key, just show me what it looks like, so something like XXXXX-XXXXX-XXXXX. Steam keys have a certain possible format to them, so if you're entering it wrong, or you're entering the wrong key, we can see.

and not only my friend and not only