Where can I find the maps/mods because they aren't in my Call of Duty 4 - Modern Warfare folder...

Every time I type /map mp_gap_training into the console is loads the map but when it says settings up game it gives the error 'file name/sum mismatch or something like that. My friend told me to join a codjumper server with a custom map on it to get the usermaps folder but idk how to join a codjumpe...