Find the desktop icon (or make one if you don't have one), right click it and go to properties.
Change the file path to:
"C:\Games\Ubisoft\Crytek\Far Cry\Bin32\FarCry.exe" -devmode
Obviously, if your farcry isn't in c:\Games like mine is, change it accordingly.
If you run the game by clicking 'play' on the popup box when you insert the CD, the cheats won't work. You need to run the game by clicking the desktop icon you just modified.