I have the one from pets at home, and I did exactly what new2cats did, I put the bottom half down first (with a bit of old litter in), then I put the lid on a few days later shes just got used to that so the next step for me is to put the door on.
They really are great, and because mabel is still a kitten she has to jump out and she manages to kick the litter off as she jumps, so no litter outside the box.

Nula