I bought my bro CoD4 and he got a key send but they now say this; Activate directly through your game! Do not use on Steam.
What is the best way to install this game?
I have an original copy of CoD4, can he use that for installing or is there a faster way?
I think that is how it's intended to be registered. You install with an existing copy (eg a friends disk), or internet image and use your key code to allow you to play the game.
I've not used them myself as I like steam too much. I've been tempted by some of the ones you can register through steam but nothing has convinced me to take the plunge just yet.
I've bought one which can be installed through Steam (L4D for my twinbro), and didn't have any problems with it.