Try following a Big Daddy until it reaches a vent and it will call for a little sister. If you have not rescued every little sister on the level, then a little sister will crawl out the vent and you will have your shot at rescuing her.
You will not find a Little Sister until you go into the Poseidon Plaza portion of Fort Frolic. If you're still wandering around the main atrium with the stage and the stairway to Fleet Hall, they will not appear there. Go to Posideon Plaza after Sander Cohen
opens the door for you and you will find them there. Once you return from Poseidon Plaza, you can find them in the main atrium as well.
There are only 3 Little Sisters in Fort Frolic. if you've somehow managed to get 3/4, then you've triggered the Little Sister Bug. If she's already gone in the wall, she's not coming back out. Details are here
http://bioshock.wikia.com/wiki/Extra_Little_Sister_Bug. Don't worry about it. If you've found 3 of them, you're good to go.
Hmmm. It's possible you missed one somewhere else, then. When I played BioShock the first time, I accidently triggered the Extra Little Sister Bug in Neptune's Bounty and spent hours trying to find out where the mysterious 4th LIttle Sister went to. Eventually,
I gave up and went on, and I got the achievement regardless. If you have a save back in Point Prometheus (before you go to the Testing Grounds leading to Fontaine), I'd recommend backtracking with the Rapture Metro to the other levels and see if you left one
in an earlier place. If not, if have an earlier save in Fort Frolic or Arcadia, you'd have to play the game going forward from there.
EDIT: Your profile says that you've gotten the achievement already, though, so feel free to ignore the above. ;)