When .pkg Files Fail to Open on Mac

If you're prompted  to "choose an application" when trying to install .pkg or .mpkg files, then those types of files have probably become disassociated from the Installer app. This may prevent you from installing new or updated software on your Mac. Next time your Mac asks you to choose an app to open a .pkg or .mpkg file, browse to the Installer app located in Mac HD/System/Library/Core Services.

To permanently fix the file association, right click on a .pkg or a .mpkg file and select Get Info. Go to the Open with pane and choose the Installer as the correct app to open these files. Then press the Change All button.