Kurzbeschreibung:

Install this addon to the same folder, where singleplayer game is installed
Run it from desktop icon or from Steam.exe in game root (its not original Steam.exe, just the launcher
Select game mode by clicking YES (for MP) or NO (for Zombies)
After alert window, close your Steam application and run it again. Dont worry, you need to do it once
With Steam running in background, run the game again and you can play MP or Zombies with bots (use Local Game menu). For playing with real players via LAN, read instructions below
Be advised to use some fake Steam account for this, just in case