Here's what to do to have a problem-free zombie game installation.
1. Download Gamemode_Zombie and put it in your Add-Ons folder.
2. Start Blockland and check it off in the add-ons menu.
3. Start a game on Slate.
4. When you spawn, select a zombie from a vehicle or zombie spawn brick.
5. Start a minigame as a Survivor playertype and shoot the zombie.
You might be putting the spawn in a place where it is partly buried, so just load Slate to see if that is the problem.