The 'next' button is disabled unless several conditions are met (in Arduino directory, with a supported version of Arduino). As I result, I spent quite a while clicking around to different directories trying to get the Next button to enable when it turned out that the issue was that I was running Arduino 1.0.3 and needed to upgrade to Arduino 1.0.5. I'd suggest that the installer should tell users when they're in an Arduino directory but it needs to be upgraded. And if there are other things the installer checks for, those should be visible as well.