Once you feed the baby, you need to help them pee and poo. Take a soft cottonball and rub down in both "areas" if you know what I mean. Eventually he/she will pee in the ball.

I'm almost positive she has worms. Most kittens are born with them. She needs to see a vet to determine how old she is, get her first round of shots and some dewormer. He'll give you instructions about feeding a baby. They need to eat about every 2-3 hours. They also need to be made to go pee and poo again. I know, it's a vicious cyclle.

Good luck. If you have any questions, there are TONS of people involved in rescue and can help you out, including myself.