You could also try enabling the debug menu for the App Store, using the terminal command below:
In there it gives you the option to delete cookies and reset the app... This might help you.
Nobody knows for sure what these options do, but it has never caused me any problems. However, be very careful not to click around too much, it may damage your machine.
- Quit App Store.app
- Open Terminal
- Type "defaults write com.apple.appstore ShowDebugMenu -bool true" (without the quotations)
- Hit ENTER
- Relaunch App Store.app
- Locate Menu and select the appropriate topic
- Close & relaunch App Store.
Fingers crossed it will work for you.