Modding the Steam version is not as simple as modding the real game. You can get a copy of the real game (download or DVD) from Amazon for about $10. IMO, that's the easiest way. If you want to make what you have work, there are a number of tutorials by Desertstriker on how to setup and mod the Steam game.
http://www.subsim.com/radioroom/showthread.php?t=208216