Just installed KiCad 5.1.6 on MacOS 10.15. Installer would not let me drag kicad to /Library/Application Support, so I did the copy with finder. No prior installation.
When I open the Symbol Library Browser it is empty, though Preferences > Configure paths indicates symbols are in /Library/Application Support/kicad/library. I removed the Mac quarantine attribute recursively on the kicad directory (which also has all x and r permissions granted), restarted KiCad, and I still have no symbols, though there is plenty in the kicad/library directory.