It could be your security settings
I had to Allow:
- scripting (NoScript plug-in for Firefox)
cookies (Zone Alarm firewall)
web bugs (Zone Alarm firewall)
mobile code (Zone Alarm firewall)
private headers (Zone Alarm firewall)
make this a "trusted site" (Windows internet options)
I'm not sure which of the above, was the problem, but after I loosened the settings for this site, the download button worked fine..