Solved after calling Tech Support, and am posting the answer in case it helps anyone else:
For whatever reason (and this didn't happen when I installed Bento yesterday), but the installer .mpkg file was set to "Open with..." Terminal. If this happens to you, you'll need to open with the Installer application that's actually built into Mac OS X.
How to navigate to the Installer application:
- For Mac OS X versions prior to 10.5: /Applications/Utilities/Installer.app.
- For Mac OS X version 10.5 and later: /System/Library/CoreServices/Installer.app.
Hope this helps!