Click on the link for Mac OS X as shown previously on the setup web page. On the Mac installation page, follow the seven simple steps to install App Inventor on your Mac. The splash page that comes up after download is shown in the following screenshot, and the 1.1 installer is still current as of this writing, but, naturally, the version number is subject to change.
Note
One troubleshooting tip: in most cases, as is pointed out on the Mac installation page, App Inventor should be able to locate the installed software on its own. But, if it asks you to type in the location to look for it, the pathname to enter is:
/Applications/Appinventor/commands-for-Appinventor
Mac folks may now skip ahead to the section about setting up and running the emulator.