Do you know what's causing this? Megacolon...other?
If she won't eat the Laxatone, rub a dose on the top of her two front paws and she'll lick it off.
Add water to her canned food, even the juice from water packed tuna.
But before any of this - vet!
Here is an article with info:
http://www.allergicpet.com/cat_constipation.html