May I suggest
If the metadata is still in the “Plex” share, then please
- 
Unininstall the package with the ERASE option (to remove the errant DSM 7 installation) 
- 
Now install any version of PMS from Plex.tv/downloads 
- 
Migration will automatically start. 
- 
it will sit at “Installing” until done. Don’t restart the NAS until it’s done. 
If DSM tells you "Operation Failed’, ignore it.
Open Resource Monitor → Processes (or Task Manager)  and look for PlexMediaServer (the new name).
Also,  look in the Plex shared folder  for Migration.log which will chronicle the progress  (takes about 2 minutes to show up after you start installing)