Have you installed Microsoft Silverlight 3? This is what is used to run the Photosynth viewer here on the site.
(If your computer is using Windows or MacOSX then download the Silverlight installer here: http://www.microsoft.com/silverlight/get-started/install/default.aspx and then double click the installer when it finishes downloading.)
Or, if you are using Linux, are you running the Moonlight 2 Beta?
(Available here: http://www.go-mono.com/moonlight-beta/ )
It might not load the Photosynth viewer until Novell releases the full version of Moonlight 2.0 and I'm not sure when that will finally be, but they're close.