The dolphin feeding pool is in the Key West area. Enter the park and bear left until you get there, it's hard to miss.
They have feeding times scattered throughout the day, typically coinciding with the end of the Blue Horizon show nextdoor I've noticed. I believe the fish trays are up to $6 a piece now, with three fish or so in each one.
Get there early, as the line fills up quick and they only feed a certain amount at each session.
If you're very, very patient.. and very, very calm.. and NOT splashing the water or waving your hands... you can generally get some petting in eventually without paying for fish. But buying the fish is a pretty reliable way of getting to touch the dolphins.
They also have a more expensive Dolphin Tour, which allows a less frenzied.. less other-tourist-elbows-in-the-ribs experience. I think that tour is around $50 or so, but I've never done it.
=)