Remove these two files, which are located in the User/Library/Application Support/Steam/steamapps/common/COD:MW2\zone\<language> folder;
OSX 10.7-10.9 now hides this directory, so please press cmd+shift+G to bring up a search window in Finder and type in '~/library' to access this directory.
Make sure you have closed Steam before you remove these two files. Boot up Steam and run Call of Duty: Modern Warfare 2 - Multiplayer. If any error pops up, try verifying Game Cache after removing these files.
Please right click (ctrl+click) Modern Warfare 2 from the Steam Library menu and select Properties. Under the Local Files tab, select Verify Integrity of Game Cache. Once that completes, relaunch the game.