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.
I just tried now, and the customizer opened okay. Maybe Thingiverse was doing maintenance or something.
If the problem persists, give Thingiverse a shout to see if they have any ideas. Alternatively, download the .scad file, and open it directly in OpenSCAD. OpenSCAD works better than the Thingiverse customizer anyway. Plus it has a code editor, in the event that you'd want to make your own modifications to the code.