i know 2 ways to spawn zombies:
1st is using the event tab . set up a time (day or hour) or a condition and then scrol down until you see the generate units tab. (the °5)

the second is with a custom scrip using generate units. you need condition in order for it to work, like a "script on entry" zone, or a script when interact with entity, like sript on joining, or script on enteraaction, or even though the custom command button on the command center.

Both require a spawnpoint, and some extra info about what zombies, how many, ect.

i hope this help