You need to give them a little encouragement, by showing them where the box is, a few times a day, and they'll pick up on it. Hope this helps, good luck.
Do Mother Cats Litter Train Their Kittens?
Make sure the cat knows that whats it's doing is bad when it goes outside the box, don't hurt it, just clap your hands or say no. Then put it in the box, hopefully you can catch it before it goes or as it is, that way it will realize where it is supposed to go. It will take a few tries, and a lot of encouragement on your part, giving it a treat once it goes in the litter box may help it understand that it was good. My parents where old fashioned.(And though I view this as cruel. It does work.) They would catch the cat 'in the act' and put it in the litter box, to finish. Or if they found a spot the cat urinated in, they would put the cats nose to it, for a minute then put them in the litter box. This idea is not he best, and is mean, the cat will probably not like you much either after wards, and should only be used as a last resort, but it works. Unfourtunately. Just do something the cat doesn't like smack your hand with a rolled newspaper, the sound will scare them. Or use a spray bottle, as cats hate water. Anything they view as bad, they will see that something bad happens when they have an accident, and once they learn where to go, they will see that you don't punish them, and this will help. Also watching the mother will help as they learn from her. Keep in mind that they will have a couple accidents when they're little because they wont be able to get there fast enough. Don't be to harsh.