You should properly indent lines, it's messy like that.

'ismaster' returns 1 even if you are an admin, you should remember that.
'start' should be placed outside the GUI. I also improved it a bit:

start = [
	cleargui
	if (! (ismaster (getclientnum))) [echo "^f3You must be master or admin to start the match"]
	if (> (indexof $zombmodes $zombmode) -1) [$zombmode $zombmap] [
		error (concat "^f3Invalid team mode:" $zombmode)
	]
	botbalance 0
	team good
	loop i $zombnum [addbot $zombskill]
	sleep 1000 [
		looplist cn (listclients) [
			setteam $cn (? (isai $cn) evil good)
		]
	]
]

Can the bots only use chainsaws? Do they go slower? Based on what you said, it looks like bots can use guns. It's no fun with just an online bot-match. The going slower thing would be cool, as it would be like real zombies...