I also have this zombie mod, but can't get it too work either!
What it claims you have to do, is adda game logic named "server".
If you go to add unit, and where it says "west" scroll down to came logic.
Then where you would normally set the unit type find object, and then in the unit selection choose your difficulty level and name it "server"
This is what it explains to do, but in a very brief and difficult to understand form. At least that is my interpretation. I tried this, but still can't get it too work! Try it out for yourself, it may work!