The ONLY way to report a player on PC is through steam, while your are in game do a SHIFT-TAB to bring up steam overlay, at the bottom leftish middle of the screen you will see players, click it and a box will appear, scroll thru the current players list or recent games list if the match has finished and find the cheater you want to report.
Click on his/her/its name to go their profile, once you get their profile page uploaded it will say either private, for friends only, or this player has not set up their profile, look to the top right you will see a button report, click it and you will have options to either report the profile for abusive behaviour or whatever, the last button in the list is report for cheating, click it, write something like "cheating in MW3" then validate it, steam will thank you for taking the time to report this person and all is done.
The one thing about the above solution is that VAC is a fail anti-cheat program as it doesn't instant ban and on MW3 p2p networking VAC just isnt set up for this kind of gaming, all you will get is the satifaction of reporting a cheater.
Your better off bombarding 402 on Twitter to get his console finger out of his xbox disk slot and put a priority on the cheating on MW3 PC as I can see that we will be heading for another MW2 hackfest and atm we already are.
Hope that helped.
