Some cats cover and some don't. But, you can try letting the cat watch you cover her stuff and she might decide to join in. Another suggestion is to take her paw (without freaking her out) and show her how to cover.
Do you have an open box? If so, you may want to see if she'll use a closed box - at least that should cut down on smell and you won't have to look at the mess
Also, do you clean/change the litter often? Some cats are very picky. Maybe your cat doesn't want to dig in a non-fresh box.
But, I'd try to fixes above first. Also, you might try searching the forums for a similar topic. I seem to remember this issue being discussed before.