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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.

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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.