MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
Ok ... That's something that Thingiverse and Makerbot should have made clear.
Recent browsers do deactivate by default 3rd party cookies (and there's some good reason for).
That's the reason for the safari error message. To solve the problem all you have to do is add a 3rd party cookie exception for "customizer.makerbot.com".
Probably too complicated for Thingiverse and makerbot to discuss about this issue.