Zombies do not appear, not at 19 not at 18, reinstalled the game, windows too, everything was checked all inclusive in the kirt settings. I do not know what to do...
Klick on the red link above that say "Updating to Alpha 19 instructions". Follow the advice "Clean out A18 data" . Then create a new world where you put a random word into the "world seed" tab.
If that doesn't solve it, come back here and post your logfile (instructions are in the sticky thread that says "Read before posting")