Open up your Hitman.ini file located in your hitman directory. Add the line 'enableconsole 1' to the file. When playing a mission simply hit the '~' key. Types any of these codes for the desired effect:

Cheat Code - Granted Wish

god 1 - God Mode
giveall - All Weapons with Max Ammo
infammo - Unlimited Ammo
invisible - 1 Invisibility