Open the file "environment.cfg".
Add the following line to the file:

CON_ENABLE=1

Save the file. While playing press ~ to bring down the
console, then type any of the following:

CODE                   RESULT
cg_menu "cheatmenu"  - Cheat Menu (God Mode/Win Mission)
cmd giveall          - Give All
cmd noclip           - No Clipping Mode