Pre-built Tahoe-LAFS ".pkg" installers for OS-X are generated with each source-code commit. These installers offer an easy way to get Tahoe and all its dependencies installed on your Mac. They do not yet provide a double-clickable application: after installation, you will have a "tahoe" command-line tool, which you can use from a shell (a Terminal window) just as if you'd installed from source.
Installers are available from this directory:
https://tahoe-lafs.org/source/tahoe-lafs/tarballs/OS-X-packages/
Download the latest .pkg file to your computer and double-click on it. This
will install to /Applications/tahoe.app, however the app icon there is not
how you use Tahoe (launching it will get you a dialog box with a reminder to
use Terminal). /Applications/tahoe.app/bin/tahoe is the executable. The
next shell you start ought to have that directory in your $PATH (thanks to a
file in /etc/paths.d/), unless your .profile overrides it.
Tahoe-LAFS is also easy to install with pip, as described in the README.