OS X 10.6.8 App Store "Get" Button Does Nothing
@rogerdpack answered this in a comment at Mac OS X 10.6.8 App Store not working:

To get around this problem, manually go to "software update", install all available updates, reboot, and then the app store GET button should work again.

If this still fails, try to reboot again, and/or to log in to the App Store before clicking the GET button.