To use cheats you must enable console from the options menu. During game, press tilde key (~) to call console window. Enter this command devmap mak to enable cheat mode. After the cheat mode has been enabled, you can use these:
god - Activate God mode (immortality)
give all - Get all weapons
notarget - Disable AI (enemies won't attack you)
mapname - List maps
noclip - No clipping mode
map [map name] - Change map (level) with cheats disabled (will have to do a devmap mak to activate cheats).
devmap [map name] - Change map (level) and automatically activate cheats for loaded map.
Maps list (can be listed in console with mapname command): mak, pel1, pel2, sniper, see1, pel1a, pel1b, see2, ber1, ber2, pby_fly, oki2, oki3, ber3, ber3b, nazi_zombie_prototype