I think you may have to bottle feed this kitten - if your mama cat rejected it, it is because she can sense that either it is not hers (s/he would not smell like hers) or that she thinks the little one is ill. Also, she may be finished with hers and just fatigued. Maybe she has no milk left? There could be a myriad of reasons.
and follow the directions for feeding baby kittens. Do you have any idea how old this baby is? Are the eyes open yet? Those sites can provide info. I have been thru bottle feeding kittens and it is a labour of love but must be done at regular intervals.
Kittens under 3 weeks have probs with their body temperatureso it is VERY important you keep her/him warm. Also, if you can find a snugglekitty so this baby stays warm, that would be good. You should make sure s.he is warm, however you do it. Holding it against you is one way but snugglekitties are wonderful!!
There are recipes for kitten glop on those sites. Do NOT give her cow's mil!! And you have to help her eliminate after she eats.
You could alternately contact a local Cat Rescue group and see if they have mother cats who would accept a new one. Even so, someone will likely have to ensure this kitten is fed and helped to eliminate.