Another unmissable story achievement at the end of the DLC, just talk to Frank-N-Builder after you finish the Monster Maker. Don't know why anyone else didn't type this up before, but here it is.
EDIT: PrimeBigTime pointed out below that you get the toadstool ointment at the start, which seems to have gone missing for at least one player, but you obtain it from the witch when you build her shack for her.