I've had the same problem and asked the question on stackoverflowasked the question on stackoverflow. As I can see on your site, you hadn't change the pathes in your project.
So, you must copy /apache-conf/qgis-web-client.conf.tmpl to .../apache-conf/qgis-web-client.conf (or rename...but I don't want to risk, so I've just copied it). Then you must replace all <path-to-qgis-server-projects> in .../apache-conf/qgis-web-client.conf.tmpl with your real projects path (for example, .../projects). Then you must copy one of .../site/js/GlobalOptions.js.templ-4326 or .../site/js/GlobalOptions.js.templ-900913 (for SRID=4326 and SRID=900913 projections respectively) to .../site/js/GlobalOptions.js.
And replace all /home/web/qgis-web-client entries with your real project path in files: .../site/index.html, .../site/index.xml.tmpl (if you will use it...or just in case), .../site/js/GISProjectListing.js (this file containts pathes to your HelloWorld and NaturalEarth projects) and in .../apache-conf/qgis-web-client.conf too.