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.

Download Now

Report as inappropriate

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.