The first time that you use Apple’s Developer Certificate Utility to request an OS X signing certificate, you may find that the “Create” button doesn’t do anything, or is disabled. This happens even if you follow the instructions at the top of the screen to install the two required intermediate certificates (click image to enlarge):
This happened to me using Safari on Mountain Lion. I solved the problem by relaunching Apple’s web app. I quit Safari, went back to the Developer site, etc. It’s an annoying problem because Apple does not instruct us to re-launch the app, nor does it give an error message. I also could not turn up any solutions by googling. But once you re-launch, everything will work fine.