Adjust the "Friends" and "Enemies" sliders to fill the lobby. Difficulty: Set from Recruit to Veteran.
Here’s a concise, informative guide on how to play multiplayer offline on PC. how to play call of duty black ops 1 multiplayer offline pc
But if you want offline (Two players on one PC)? Bad news: The PC version of Black Ops 1 famously removed split-screen zombies. You cannot do it without third-party mods (like the "Black Ops 1 Reimagined" mod, which is complex to install). Adjust the "Friends" and "Enemies" sliders to fill the lobby