Instructions
Open the config.cfg file of the game in the "Players" directory with a text editing program. If you are using Windows, this will likely be Notepad.
Add: seta thereisacow "1337" at the end of this file.
Select a cheat and key and add those after "1337" in the format: bind [key] [cheat]. For example, if you want to use the "notarget" cheat, where enemies ignore you, with the "Q" key, enter: bind Q notarget.
Save the file then use the right-click button on the mouse and choose "Properties." Set the file so that it is "read only."
Start a game and select a mission. Test the cheat by hitting the appropriate key. You will not get a confirmation sound, but you should quickly see the effect as you play through the level.