easy peasy, go to your mods section of A3, if not subscribed to the ryanzombies (gag mod) then sub it. if sub already, unsub and re-sub. (this fixes issues 90% of the time)
if that doesn't work go to "folder" check if the mod is actually there, and last of the last resort
download the file and stick the files in the folder that A3L sees as the modded folder for Ryan's zombies.