To use cheats, pause the game (press ESC) and then just enter them: 
All Modes: 

Code   	Result	
painless    	God Mode	
fogging	Fogging on/off	
followme   	Move Camera	

Sharpshooting Mode: 
Code  	Result	
weapons    	All Weapons	
ammo 	Unlimited Ammo	
autoreload 	Autoreload on/off	
autofire     	Autofire on/off	
slowmo	Enemies are Slower	
slowrocket  	Rockets are Slower	

Driving Mode: 
Code  	Result	
ghost	Drive Through Walls	
nitro	Unlimited Nitro	
freeze    	Toggle Timelimit	
chantastic 	Drive Faster	
snow  	Snow	
susonly     	Drive Only With Tires	

Action/Adventure Mode: 
Code  	Result	
weapons	    All Weapons	
ammo  	Unlimited Ammo	
freeze     	Freeze Enemies	
targeting   	Toggle Auto Targeting	
laser 	Toggle Laser Targeting	
ghost	Walk Through Walls	
followterrain     Jump as High as You Want	
mrbones	Become a Skeleton	
shocked	    Become Electric	
fps   	First Person Perspective	
fragyuck   	More Blood	
pillowmode  	Small Heads	
bighead     	Big Heads