Problem: I had a strange problem after upgrading to Lion (OS X 10.7). Any .mpkg files wouldn’t open Installer but instead would open Terminal, and no installation would happen at all. I saw someone else post that updating to 10.7.1 would fix it. For me, this didn’t work. Whatever happened to me, it happened after the update.
Clue: The problem is noticeable in Finder, as I could see that the file extension icon was not the gold/tan box icon that goes with Installer. That’s not good. It means that Installer.app is no longer the default application for the file extension.
Solution: There are two ways to do this.
1) In Finder, right-click, or get to the property window, and “Open With” and navigate to /System/Library/CoreServices/Installer.app. There is a checkbox to always open with this app. Check it, and let Installer run.
2) Open Installer itself and drag the .mpkg file into it. Run Installer.
Both of these options will cause Installer.app to run, and saves the new app default back to Installer. To check that it’s working, go back into Finder, and view the file extension. If you see the gold/tan icon on the file, it’s fixed.