This solved it for me:
1. If it's open, close the appstore / itunes
2. Go to your "Library"...
> Open Finder
> Hold down 'Option'
> Click Library
3. Delete some files (cache/com.apple.appstore, cookies/com.apple.appstore.plist) associated with the app store:
> Open folder 'cache'
> Drag 'com.apple.appstore' to the trash
> Up one folder to 'library'
> Open folder 'cookies'
> Drag 'com.apple.appstore.plist' to the trash
> Empty trash
4. You may need to restart at this point. After the reboot, try opening the appstore and downloading. Worked for me at this point.