JoomGallery installs "successfully" but won't run.

I'm using Joomla! 3.3.3. I installed JoomGallery (http://www.en.joomgallery.net/), then clicked its "Start Now" (or something) button. For several minutes I saw the "in progress" icon spinning in the browser tab. Ultimately, I got a 404 error. I was never able to use JoomGallery. In installed/installed three times. Tech support seemed stumped and they sent me here.

Any ideas?

Thanks.