To enable cheats, you need to modify the "freedom.ini" file in the game folder (create a backup copy of the file before proceeding). Open the file using Notepad and add the following line: 

EnableConsole 1 

Then, while playing press [~] to display the console window and enter one of the following codes to activate the corresponding cheat function: 

god 1 - God Mode 
giveall - All Weapons 
infammo - Unlimited Ammo 
flymo - Fly
forcemult 
giveall - Givesall
giveneeded - Give needed for level
globals
god - God
goto (map) - Goto map, where is enter map name
herocoords
hira
hiraclip - Clipping Mode
infammo - Unlimited Ammo
invisable - Invisable
nailgun - Give Nailgun
no_damage - No Damage
no_damagefriendly
autoaimangle
osd
popstar
radar - Give radar
reload - Reloads Weapon
showheroroom
dumpeventhem
blockfire - Blokes Enemie Fire

During gameplay quickly enter the following codes:

Code Effect 
IOICHARISMA Charisma 
IOIFASTMO Fast motion 
IOIHGUN Get handgun 
IOIRIFLE Get rifle 
IOIROCKET Get rocket launcher 
IOISHOTGUN Get shotgun 
IOISNIPER Get sniper rifle 
IOIBLIND Invisable Mode 
IOIFLYMO Ragdoll flymo 
IOISLOWMO Slow motion 
IOIAMMO Unlimited ammo 
IOIGOD God Mode 
Note: Weapons cheats also gives you other things.