Software install does not finish
I recently went into the Mac App Store and saw that I had an update for an app: "Significator for iTunes." I proceeded to update the app and it was able to download the update however in the Mac App Store, it is currently stuck at "Installing - less than a minute." It has been stuck at this step for the past 2 days and I have been unable to shut down or restart my computer because of this.
I also receive this message when trying to shut down or restart:
This prompt has also been unresponsive for the past 2 days.
I thought I would be able to cancel this by going into Activity Monitor and force quitting the process but after doing so, I still cannot restart or shut down.
Any ideas or suggestions?
MacBook Pro (Retina, Mid 2012), OS X Mountain Lion (10.8.1)