Mountain Lion Doesn't Download from Store
Try one of the following solutions: 


Quit the App Store and reboot your Mac. Most of the times, your problem will be solved.
Try again in 15 or 30 minutes. Sometimes, as large as the server farms might be, they simply can't handle the overload in request. Maybe later, there will be an open slot for you
Flush the cache of the App Store. Opeq Finder and use the "Go to" option to go to ~/Library/Caches/com.apple.appstore. Remove the file and make sure you've emptied your trash. 


I hope one of these things will help you out.