Hi Jim,
SuperSynth was added into the development builds in August of last year, and was released as a part of FreeSurfer 8.2.0.
Builds can be downloaded from this page:
https://surfer.nmr.mgh.harvard.edu/fswiki/rel7downloads
There has also been a patch for the release, instructions for applying the patch are available
here.
I would advise you to apply the patch, as it should contain updates that will have an impact on SuperSynth.
Best,
Jackson