How to get baby bunny to poop?

the other day i killed a rabbit, when i was gutting it i found 2 baby bunnies inside, one was dead the other was alive,we took it out of a little sack that is was in and took it home. we have been feeding it a type of powered milk for animals, it hasnt pooped yet but it has only been one day that we have had it. how long till it poops or how do i get it to poop?