Hi there - the theme previewer will only work if you have content on your front page. Your front page was giving a "404 not found" error since no page had been assigned under Settings > Reading, so the design tools weren't loading. I created a temporary homepage for you and set it up under Settings > Reading, as you can see here:
http://cl.ly/image/0p3Y0A0Z073P
Please let me know if you can load the design tools - thanks!