Google has very few links to people who have had the problem, and the only answer seems to be that you may have downloaded the checked build, and you need to chose the Ultimate version; I had chosen Business edition.
Chris responds: You are probably installing from the "checked" ISO image. Please choose "Ultimate" and your key should work.However, when you are on that screen there is no way to go back, do I don't know how you would select Ultimate.
Anyway, the solution that worked for me was to simply leave the field empty, yup, that simple.
So, if you downloaded the checked build, chose the Ultimate version; if you're stuck in that page, I hope this helps you