I figured this out and got it working! I believe this may have been related to the fact that I had previously installed the Synthesizer V free version and then purchased and installed the pro version without first uninstalling the free one. Below is what I did,

1) downloaded a fresh copy of Synthesizer V Studio Pro (same version as I already had installed)
2) deleted the Synthesizer V VST files from my computer
3) reinstalled Synthesizer V Studio Pro including the VSTs (I did NOT uninstall or deactivate anything first)
4) went into Reaper and forced a complete rescan of all VST folders
5) NOW IT WORKS!

My takeaway/advice is this, if you install the free version and subsequently buy the pro version be sure you uninstall the free one BEFORE you install the pro one!