KiCad for OS X is available as two disk images (dmg). You usually want to install both.
KiCad is supported on OS X 10.7 through OS X 10.12.
KiCad main package contains the KiCad suite, documentation, schematic symbols, translations, and some templates. It gets footprints from the internet.
KiCad extras contains the footprints and all the instructions you need for making printed circuit board layouts while offline.
The nightly builds are snapshots of the codebase at a specific time. They may contain more bugs than usual, although we try our best. Use them for testing the newest features: