If you run in verbose mode you should see a successful OSC update request. If you do and still no GUI, it maybe something else.

jack-dssi-host -v whysynth.so
<snip>
WhySynth_gtk starting (pid 4176)...
jack-dssi-host: OSC: got update request from <osc.udp://fedora18:14218/dssi/whysynth/WhySynth/chan00>