You need a DSSI host, like Rosegarden, MusE, Om...
(I advice you not to use jack-dssi-host as it does not accept in input
sysex messages yet). Please note that if you use Rosegarden you won't be able
send patches to DSynkant due to the fact that
Rosegarden loads only the first sysex in a file and disgards the others.
I you use Rosegarden you will have to use an external sysex sender program
like simplesysexxer or MusE.
For the moment there is no GUI at all, everything is controlled by MIDI messages. If you want to edit D-50 patches you can use UniQuest VC-1 (but it costs money) and D50 Virtual Editor (there is a free version but slightly imcomplete, although usable). Maybe I will make an editor but it is not my prior goal.