I am trying to upgrade to OS X Mavericks from 10.7.5 but every time I click the install button in the Mac App Store, it looks like nothing is happening and now the button is all greyed out and I can't do anything.
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You can enable the Debug menu by typing the following into Terminal:
defaults write com.apple.appstore ShowDebugMenu -bool true
You can then clear the cookies using the Clear Cookies option in the Debug menu.
In addition, you can reset the application using the Reset Application option.
You can disable the Debug menu using the following:
defaults write com.apple.appstore ShowDebugMenu -bool false
I'm posting this due to having the same issue and being frustrated not being able to find a clear solution online.
I had the bug where the AppStore just said "Install App" or "Upgrade Free" and clicking the button would do absolutely nothing.
The fix was to reboot into safe mode, and then reboot again back to normal mode.
Reboot your mac, right as the startup chime happens, but before the Apple logo shows up, hold down the shift key. This will boot into safe mode and will take about 2-3x longer to boot up than normal.
Once it's finished booting up, just reboot again immediately, back to normal mode.
Now the app store should work correctly again and prompt to download Mavericks, rather than install it.
If you accidentally close Launchpad (which pops up and shows the progress of the download) - use CMD+Space and search for & run Launchpad.
Hope this helps others out there.