Mainstage 3 installation hangs at "Basic Content Download"
I purchased Mainstage 3 today from the App Store. I am attempting to install it on a MacBook Air (mid-2011 model). I have upgraded to current Mtn Lion OS. I have 30 GB of free disk space. The app has downloaded but it's trapped on the Basic Content Download dialog. It forever says "Installing..." but there is no activity in the progress bar. I have tried:
1. Canceling install. Restarting. Trying install again.
2. Canceling install. Throwing out Mainstage from Applications folder. Restarting. Trying install again.
3. Canceling install. Signing out of App Store. Restarting. Signing back in to App Store. Trying install again.
4. Canceling install. Signing out of App Store. Throwing out Mainstage from Applications folder. Restarting. Signing back in to App Store. Trying install again.
I'm officially out of ideas, short of taking it to the Genius Bar, which I don't want to have to do if I can help it.
Any suggestions?
Thanks!
MainStage, OS X Mountain Lion (10.8.4)