To use the codes type them in at the console.  Press the ~ key 
to bring up the console. 

impulse 11     Invulnerability
ode to jack     Kill all enemies
impulse 10     Big magic attack
fly     Fly mode toggle (press jump key to fly)
dir maps     List available levels
map x     (Where x is the level name) Level warp 
noclip     No clipping mode toggle
impulse 43     Suicide
impulse 69     Another suicide