I stumbled onto this 'solution' - tried it on a Win7 system and it worked great - I tried to enable cheats from the console (~) but got the can't use now message so I tried this solution and it worked for me - ========================================================= [~], tilde button), then enter "sv_cheats 1" to enable cheat mode.
If the message: "Can't use cheats now; please exit to main menu and start your own listen server with "map mapname" so that you could enable cheats." appears,
enable the map c5m1_waterfront or map c5m2_park code. You will return to the main menu. Enable either map code again, then enable the sv_cheats 1 code again. You can now enter one of the following cheat codes: god [0 or 1] - Toggle God mode for entire team give health - Gain health give defibrilator - Defibrillator give adrenaline - Adrenaline give first_aid_kit - First aid kit give pain_pills - Pain Pills kill - Commit suicide quit - Quit game noclip - Toggle no clipping mode upgrade_add incendiary_ammo - Toggle Incendiary Ammo upgrade upgrade_add explosive_ammo - Toggle Explosive Ammo upgrade upgrade_add laser_sight - Toggle Laser Sight upgrade give weapon_grenade_launcher - Grenade launcher give melee - Hunter claws give rifle_desert - Combat rifle give rifle_ak47 - AK-47 give sniper_military - Sniper rifle give pistol_magnum - Magnum give smg_silenced - Silenced SMG give shotgun_chrome - Chrome shotgun give shotgun_spas - SPAS shotgun give tonfa - Nightstick give vomitjar - Boomer Bile give electric_guitar - Guitar give frying_pan - Frying pan give machete - Machete give pistol - Double pistol give pumpshotgun - Pump shotgun give autoshotgun - Auto shotgun give smg - SMG give rifle - M16 give hunting_rifle - Sniper rifle give molotov - Molotov ****tail give pipe_bomb - Pipe bomb give propanetank - Propane tank give gascan - Gas can give oxygentank - Oxygen tank give ammo - Refill ammunition sv_infinite_ammo [0 or 1] - Toggle unlimited ammunition z_spawn spitter - Spawn Spitter z_spawn jockey - Spawn Jockey z_spawn charger - Spawn Charger z_spawn zombie - Spawn common zombie z_spawn mob - Spawn zombie mob z_spawn boomer - Spawn Boomer z_spawn hunter - Spawn Hunter z_spawn witch - Spawn Witch z_spawn smoker - Spawn Smoker z_spawn tank - Spawn Tank
Oh - apparently a "listen server" is a type of server you can create with the game to allow mutiplayer games from your location, but also simply evokes a "cheatable" single player experience - I had not fought in the park before and it was super fun! So you don't really do anything special, other than from the main menu open the console, type in the map mapname envoke the sv_cheats 1 and any other commands now available to you.