In order to use cheat codes, you will need to edit a game file. Locate the TdInput.ini file in your My Documents\EA Games\Mirror's Edge\TdGame\Config folder and open it with Notepad. Locate the first key binding in the file which should be: 

Bindings=(Name="D",Command="GBA_StrafeRight",Control=False,Shift=False,Alt=False) 

Using this same method, bind a new key to the cheat you wish to use. For example, if you want God Mode to be activated when you press [F2], you would add the line: 

Bindings=(Name="F2",Command="god",Control=False,Shift=False,Alt=False) 

Add each new binding on a line by itself ABOVE the first binding you found in the file. Make sure you don't bind the same key to multiple commands. Save the file and then load the game. Use your new bindings to activate the cheats while playing. You may not receive any visual or audio indication that the cheats have been activated. 

This is a list of the cheat commands available: 
 

Code:                     Result:
god                        God Mode
jesus                     Jesus Mode
dropme                  Free Cam (requires God Mode)
FreeFlightCamera   Change 1st/3rd Person Camera
ammo                    Unlimited Ammo
loadfullinventory      All Weapons
stat xunit               Show FPS
slomo 0.4              Slo-Mo Mode ON
slomo 1.0              Slo-Mo Mode OFF
LockAllLevels        Lock All Levels
UnlockAllLevels     Unlock All Levels
LockAllTT             Lock All Time Trials
UnlockAllTT          Unlock All Time Trials
Leipzig                  Unknown Effect