Bring up the console with [~] then type any of the following cheat codes:
Note: To enable the console, add +set com_allowConsole 1 to your command-line shortcut for the game. Alternately, you can try using [Ctrl] + [Al + [~] to bring up the console.
Example: C:\Doom3\doom3.exe +set com_allowConsole 1
-or- if your game is stored in a folder with a space:
Example: "C:\Program Files\Doom3\doom3.exe" +set com_allowConsole 1