I have never got FluidSynth working with Jack. (I think I have once but the sound wasn't right and Jack itself was quite crashy.) I just use the ALSA back-end, which should be fine on Linux machines unless you seriously care about real-time (from what I understand).
Just use -a alsa as a command-line flag to switch to the ALSA back-end. This tends to "just work" in my experience.