Every time I have raised a litter of either motherless kittens or puppies, I have used a cotton ball with a little baby oil on it which I rub gently in a circular motion over their genitals. After a couple of minutes they always pee and/or poop - I've always found this to work. Make sure you use a cotton ball without oil afterward to wipe off the excess baby oil so the mommy doesn't lick it off.
__________________ I LOVE MY BABIES |