let it go to the loo where ever it likes then take the kitten and gently rub its nose in what it has done or at least close enough so it can smell it. Then take it to the littler tray so it knows to go there...that how rabbits work anyway :o )
There isn't much point rubbing an animals nose in its urine as animals can't reason like humans can and won't understand what you are trying to teach it.If you catch the kitten messing you need to take it to the litter tray whilst it is in the process of weeing or pooing.Cats like a private place to do their business so make sure it is not in a busy part of the house.Some cats are very fussy about having very clean litter too.You may have to experiment with different types of litter.
It might be that he doesn't like the litter you're using. Try using a very fine-grained type (Premium Choice is good) as it's softer for them to stand on.
I agree with the other posts on cat litter but why not try just newspaper in the tray for a starters, kittens love shredding and this might give him/her a kick start into using the litter tray once it starts to use it , you can gradually put cat litter in. Good luck g.
I have had a number of cats/Kittens and i have found that a higher grade cat litter helps but mainly try keeping an eye on the kitten and when you think it is looking to go place in tray also i have found putting the tray in a quiet corner with a bit of catnip in or near the tray helps.
Enjoy you new addition it will give you much pleasure and love.