This is what I did to get it working. Hope this might help someone:
- go here (Release Development build · stoermelder/vcvrack-packone · GitHub) and download the .vcvplugin file for your platform.
- move that file into your plugins folder (e.g. Documents/Rack 2/plugins)
- Make sure that you have “Add All” chosen from here (VCV Library - Plugins)
- If you have Rack open, quit it and open it again