Installing CKAN on macOS
Download CKAN.dmg from https://github.com/KSP-CKAN/CKAN/releases/latest
Drag CKAN.app to your desktop (or any other folder you prefer)
Double click it
If you are on macOS Catalina, you'll notice that CKAN's Console UI opens in a terminal, instead of the GUI. This is because macOS dropped support for 32-bit applications with version 10.15, and Mono's WinForms implementation is 32-bit only. For more information, check this PR and this issue.
Installing via Homebrew
NOTE: As of early 2019, Mono does not work when installed via Homebrew, see Homebrew/homebrew-core#35848. Use the above method to install Mono.
UPDATE: The above issue has been closed by the Homebrew team despite the problem still existing and no progress having been made toward a fix. Do not use Homebrew.