The beta doesn't have an uninstaller, Blizzard's directions are (by their own admission) not thorough. Where are all of the D3 directories and preference files, and what is the most effective way to zap them?
Uninstalling applications in Mac OS X is very different than uninstalling in a Microsoft 1Windows environment because Mac OS X has nothing like the Windows Registry.
Most Mac OS X applications (anything that ends in .app), like Safari and iTunes, are bundles. The application icon you see in Finder (typically the Applications Folder) is usually a special folder itself, made to appear as a single double-clickable file. These "folders" contain all, or at least most, of the files needed to run the application, ie they are self-contained. This means that to uninstall these applications, you only have to drag them to the Trash. If you control-click (or right-click) on an Application icon, and you see "Show Package Contents", then it is a bundle.
For Diablo III Beta, you just delete the folder, because it does not create Package Content for you to delete.
But it does create fill .txt files that are not part of the installation. Here is a screenshot of my Mac with all files with "Diablo" in their name.
And here's the screenshot with the "Beta" name search:
Aside from the Diablo III "Beta Setup" that is in my Download folder, there is "diablo3_beta_enus-driverinfo.txt" that is in /Users/Shared/Battle.Net/Client/Blizzard Launcher.1454/Support
The others files are inside the folder in the applications (or in the location you selected).
Also inside /Users/Shared, there are 2 folders called: Battle.net and Blizzard. They are created after you install any Blizzard Game like WoW or SC or Diablo. I don't recommend to your remove those folders, unless you are going to stop playing Blizzard games.
There is a file that saves your preferences called D3Prefs.txt the location is: /Application Support/Blizzard/Diablo III