Well, the basics are simple (although I don't know FL specifics) - you set up a MIDI out for the track(s) you want, and select a MIDI channel for them. If the MIDI to USB cable also works as an output device, you just plug the MIDI out cable into the MIDI in of your keyboard/sound module, and possibly set it up to receive on the same MIDI channel as you're sending out from FL. And then it should work.