Open up this document in Notepad and you'll find it's a little harder to edit than cfg files from other games (if you're in the habit of playing around with cfg files that is) because the different settings aren't separated by carriage returns.
For Steam users, this is 'C:\Program Files (x86)\Steam\steamapps\common\call of duty modern warfare 2\players\' by default. The configuration file is called config_mp.cfg, and you'll find it in main installation folder under 'players'.
