Have you verified the registration key from Amazon on Steam service?  Just asking, I'm like 99% positive you MUST go through Steam to install MW3 anyhow, sooooo... go into Steam, go to your library page, right click MW3 - it doesn't matter if it's single or multiplayer that you click, it would reverify and download what you are missing... but right click which ever one and go down to 'Properties', click the 'Local Files' tab at top, then 'Verify Integrity of Game Cache', it will check all files, then download what you are missing... as I said I'm sure it should show in Steam, BUT if it is NOT showing in your games list at all, I would definitely call Amazon support and verify your purchase data with them, name, address, credit card, purchase confirmation number, etc. make sure you verify you are talking to the right people first and don't give out that information unless they ask specifically for what they need first.  Hope this helps.