Press ~ to display the console window and type "remoteadmin 1". Now, type "LOGIN developer" to enable cheat mode and you will be able to enter the following codes:
God mode: god [0 or 1]
No clipping: noclip [0 or 1]
Flight mode: fly [0 or 1]
Infinite ammo: uammo [0 or 1]
All weapons and ammo: give_all
Display framerate: enable counter
Display crosshair: enable crosshair
Cycle through crosshair: srotate crosshair
Level select: loadscenario("umc_spscenario1","[level name]")