This part of Call of Duty is a Steam Play game. This means that you can redeem a Mac code and is then entitled to both the Mac version and the Windows version of the game download. The correct version is then depending on the operating system of the PC, which uses the Steam client will download.
Mac Steam codes' can buy Steamplay' games, depending on the current price of the game (5.97 at 04.12.2014), be much cheaper than a 'normal' Steam buying code - but it should be noted that you should first check to see if the game really is because a Steam Play game.
Also, I would like to say that for me the multiplayer mode does not work properly - but this has nothing to do with the Mac Steam code, but with the fact that Modern Warfare is no longer supported for some time by Punkbuster. Since I bought the game, however, primarily to be able to play again the single player (this time on the PC rather than on the Xbox 360), I give the game despite the multiplayer problems 5/5 stars - for a good price / performance ratio for current price.