While IW4x can run standalone if you have the full game files, owning the game on Steam is the most reliable way to ensure you have all necessary assets.
You download the IW4x client files (the "magic" that makes it all work). You drop the iw4x install
C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Modern Warfare 2 Deploy Files: While IW4x can run standalone if you have