I had similar problems trying to upgrade from 10.7.5 to Mavericks: the Mavericks icon was greyed out and the Download wasn't proceeding. This solution worked for me:
I figured out that the download was paused by going to the Launchpad, and finding the Mavericks icon, which said "Paused" underneath it (even though I don't recall pausing it myself). I then held my mouse down on the Mavericks icon until it started shaking, and then clicked the little 'x' to abort the download. I then went and plugged my MacBook Air directly into an ethernet cable, in order to have a much faster connection to the web, and restarted the download within the App Store. Again, the 'OS X Mavericks' listing in the Purchases section of the App Store turned gray, but in the Launchpad view, it showed 'Downloading' underneath the Mavericks icon and the available disk space (as viewed in the Finder window) was slowing decreasing.
I think the App Store is a bit buggy: it should properly indicate when it is actively downloading Mavericks, and should have a progress meter. It should also indicate when a download has been paused. The App Store should provide the ability to abort/restart/pause downloads.
The key to solving this was the realization that I could abort/restart the download via the Launchpad. The other key was not trying to download via the slower WiFi connection, but instead using the much faster Ethernet connection.