Just purchased Mountain Lion - not downloading

I recently purchased a Mountain Lion upgrade for my 2010 Macbook (which came bundled with Lion). I purchased the software, redeemed the code and can see the software under the MacApp "Purchased" tab. There is a "Download" button, but nothing happens when I click on it. When I click "Check for Unfinished Downloads," I get a message saying that all downloads are complete. How do I complete this download?