Cat Won't Use Cat Box?

Hang out with her in the bathroom for a while after she eats. When she starts to poop, pick her up and put her in the box. If she stays and finishes her business, reward her with kind words and petting. If she tries to get out, keep putting her back in. Do this for a few days or until she reliably starts going straight for the box.

You might also try a different kind of litter. If you're using clay or something else that doesn't do much about urine odor, you could try adding a thick layer of baking soda and mixing it in, adding a bit more each time you scoop the poop out (which should be every time you go to the bathroom).

Pine pellet bedding for horses is another option; it absorbs the urine and smells like pine trees. They sell it as cat litter, too, just for about 3 times the price.

Fishercat