The latter can be found on Harry-Mulligan-892/F at X:-14.41, Y:26.78, Z:-50.08) After this, she should be available to spawn in the swamps. Another is to build a campfire site for ghost stories. You'll have to place some charred flowers, gravestones and a bunch of iron fences, all of which can be found in the Weird Woods. (One beginning task is rebuilding her garden. To start, you'll need to Discover the Spooky Girl in her native Weird Woods and complete one of her tasks. All credit goes to gandalfwhi40 for finding this:
Update: This is pulled off of the Lego Worlds Wiki page.