How to downoad Modern Warfare 3 from Steam?

Guys, How am I suppose to download the game when I did not buy the key direct from Steam? I got the key from Ebay and this is Steam Key..Thanks
P/s : I never install or play any games from Steam.
Answer #1
Open up the steam program which will probably be in the system tray. Look at the top and click on the games tab, and then click on activate a product on Steam.
Answer #2
Okay, thank you.
Answer #3
did it work?