NZBGet is nicely packed as Mac OS X application. The installation is simple:
In the web-interface go to settings page and setup at least one news server in section NEWS-SERVERS, save settings, reload (soft-restart) NZBGet and then you can start your downloads.
In the menu (via menu bar icon) choose Show in Finder -> Post-Processing Scripts. Finder shows you the directory where NZBGet looks for post-processing scripts. Put new scripts into this directory. Refresh web-browser, the new scripts should now be shown in web-interface, available for configuration and use.