Ive added it on a fresh wipe and on an already running game and both worked. Although if you use it ona preexisting game you would either have to a) go to regions noone has currently explored yet or b)delete the region files your base isn't in and try it that way. both ways i have had it spawn though, so you should be fine.
Edit: I have no idea how to do prefabs yet, but i am starting to play with Hal's editor, so i can make an Alexandria-esqe prefab. Laz how did you make such a big prefab spawn on the ground? instead of floating?