I have to respectfully disagree with Brian on his point #1. Assuming what he's assuming, unless there is pressure pushing water vertically through the substrate, the a vapor barrier is of no use. Brian IS, however, correct in stating that the moisture is due to the humidity condensing on the cold concrete surface. Now, what to do.
Install a fan to draw the humidity out: Well, understand air in = air out. The air that's vented to the outside by the fan must come from somewhere. So you're either going to draw air from outside (which is humid and counter to what you want to do), or draw air from inside the home itself (which is probably air conditioned and not what you want to do either. Electric bill.) Equalize the temperature of the floor to that of the air: Well, not likely unless you heat the concrete. Install a de-humidifier: No, not likely. It will run all day. Raise the temperature of the porch air: Well, this should help. Warmer air holds more moisture and may warm the concrete a bit. BUT, this would reduce the benefit of having a cool porch.
Lower the temperature of the porch air: Unless you completely enclose the porch, humidity will continue to enter the porch and moisture will continue to accumulate on the floor. But by enclosing the porch, you make the room air's temperature more in line with that of the floor. This is really the only solution, plus, keeping the humidity out. If you do the complete enclosure, then a dehumidifier would be a benefit in making up the difference in taking that last bit of humidity out of the room.
In your case, you can compare rooms in your home to your porch. Rooms in the home that are enclosed and thermostatically controlled have no problem. (I'm assuming you have a dehumidifier in your cooling system.) So, in short, you'll basically need to enclose the porch and treat it like another room in the house. You'll have to insulate with insulated glass, and you'll have to insulate the concrete from the outside elements too, again, to make sure everything in the room is the same temperature (concrete included). It's a job that, should you decided to do, needs some professional thought. Best to ya.