Thank you for giving this kitten a chance at life!
I just googled caring for a newborn kitten and found this site: http://www.safehavenforcats.com/newborn.htm
You can do your own search and find other sites.

I know you need to stimulate the kitty's bottom to get them to poop. That is in the site posted above.

Also, you might take it to the vet and see what they say as well. Maybe it is sick and that's why the mom left it.
I hope that's not the case though.

Good luck!! Hopefully others will see this and give you their first hand experience. I know several people on here have had bottle babies.