Also, what has worked for me with kittens and cats(I do this whether they know about boxes or not, when they first come to my place), I gently set them in the box, take their front paws, and "help" them scratch the litter. Left paw, right paw, left paw....etc

Since this is a reflex action for covering up their poop and stuff, the penny seems to drop pretty quickly.

Let us know what happens.