You might try putting dirt in the box instead of commercial kitty sand. Or perhaps a layer of dirt over the sand. Even better, add some of the dirt from the flower pot, which will already have the smell of the kitten's doings in it. Take a minute or two to play with and pet the kitten. Then place it gently in the box and maybe even playfully take one paw and scratch it in the dirt to give the kitten the idea. While doing this, you want to be careful to make it a pleasant time for the cat. Often that's all it's taken for me to box-train a kitten. Their natural instinct is to go in the dirt (as you've seen), all you need ot do is re-orient your kitten's natural instincts to the correct place. If the kitten starts scrabbling in the box or goes in the box, praise it lavishlyand pet it. Try to watch, especially right after feeding or waking up, and if the kitten heads for the flower pot, take it instead to the box, again heaping on the praise. If the kitten is still resisting the box, you might try a different kind of sand. Some cats are picky about that. And you might invest in one of the "kitty no" products to put on the plant, to make it a less attractive sandbox for the little one. Most cats can be trained, it just takes time and patience and a lot of positive reinforcement.