Which zombie folder you need to copy depends on your Project Zomboid version.
The version is shown on the main menu in the bottom right corner.

Version 41 (Current stable version - use this if you dont know)
  -> copy the zombie folder from 41/zombie to your project zomboid installation. It will ask to replace 24 files, confirm.

Version 42.12 (Beta branch "legacy_42_12 - 42.12.X")
  -> copy the zombie folder from 42.12/zombie to your project zomboid installation. It will ask to replace 27 files, confirm.

Version 42.19 (Beta branch "unstable - Latest Build 42 - UNSTABLE - BACKUP FIRST")
  -> copy the zombie folder from 42.19/zombie to your project zomboid installation. It should not ask to replace files.

Only copy the zombie folder, NOTHING ELSE.
