First make a backup before making any changes. Open the file 'action.cfg' with any text editor. Then add the line at the top:

bind tilde console

Now during gameplay, press ~ and type any of the following "Codes":

Cheat Code - Granted Wish

toggleai - Enemy AI On/Off
toggledoors - All Doors Unlocked
toggledamage - Toggle Damage for All Characters
badguysarelousyshots.01 - Enemies Are Less Accurate
toggledetection - Enemies Can't Detect You
gimme (item) - Gives (item)