When Synth1 Librarian opens for the first time, it doesn’t know where the actual Synth1 DLL and zip/bank files are, so these settings need to be provided (use the file browser buttons circled in the image):
The plugin needs to be restarted after changing these settings (or better still, restart your DAW/host), and then you can build your patch library.
UPDATE Feb 02, 2018:
Some users have reported better success when running their DAW/host in Admin mode (i.e. “Run as Administrator“). If you run into problems initially, see if running in Admin mode helps.