The reason I am doing this believe it or not some people don't know how to activate a game on Steam. Well here is some easy steps on activating a retail or download serial number if compatible with Steam.
First off after logging into Steam press on Games circled in Red
Next press Activate a Product located in the Red circle on Steam
Next type in the serial number of the game your trying to Activate
Once you have done so your game should be activated if it doesn't activate its either cause your game is not compatible with Steam or there maybe issues with the Activation. If your having problems with the Activation visit www.steampowered.com and go to Support.