I haven't tested this, because I don't own zombie horde, but I *think* the zombies that it spawn are actually the "scarecrow" entity. So, under infliction entities you want to list it like this. The "0.5" in this case is the chance that a player will be infected when they are attacked (0.5 = 50%).
If that doesn't work, then they are actually a different entity, but I'm not sure what. I have a tool that can tell you for sure, if you're interested PM me.
Also let me know if it does work so I can get confirmation!