Sorry about that. We've updated the script (and the installation directions).
spine-scripts/gimp at master · EsotericSoftware/spine-scripts

Gimp gives me an error when I try to export to spine
"An error occurred running python-fu-spine-export
TypeError: object of type 'NoneType' has no len()"

Could you send us your Gimp project to contact@esotericsoftware.com? Please reference this forum thread.