I just put my kittens in the box to look around, and they started using it right away (still with a few accidents) But all in all, it's going very well. Maybe try putting a clump of mommy's waste in there, just so they smell what it's for....and not just a clean box with fun sand to play in! Good Luck!
Oh, forgot....if they start to eat it, use rabbit pellets, or yesterdays news as litter instead, ingesting clumping is bad. Also, the smaller the area they are confined to....the quicker they will learn.