Well, first of all, correct me if I'm wrong, but I do not believe it's supposed to shoot players, and rather zombies only. Now on to your problem. Here's a little checklist for you:
- Make sure that zombies are enabled in the first place. :P
- If you are using Rotondo's Left4Block zombie mod, make sure that in the Left4Block section of the mini-game editing menu, that Zombie Friendly Fire is enabled.
That's all I can think of right now, but try those, see if it works. If it doesn't, post here again and tell us exactly what you did after reading this and if something went wrong, what it was.
