KIEGUY  Thank you very much for this tip on getting those .js scripts actually loaded into GreaseKit in Safari! Worked for me. Another way then leading from your tip, is, of course, to right-click on the .js file and pick "Open With" > "Safari".

I've been struggling with this issue tonight and this is the first I've seen it explained, simple as it may be.

Frustrating as I was expecting a "Load Script" button in GreaseKit's "Manage Scripts..." window...and "Add" wouldn't do a thing!!!

You'd think this essential issue would be mentioned elsewhere, either on the GreaseKit site, or on the userscripts.org forums, but not in my searches. Maybe I didn't look long enough either place, but its seems like something that should come up right away.  
(Version 1.5)

KIEGUY  Same issues with me...it does not create a disk image, nor does it open one (created via Apple's Disk Utility) that I point it to manually. I have tried several times; I am pointing directly to the disk image.

Error:

"The target volume is not available, or is not available at the same path as when originally chosen.

Please attach the target volume or recreate the scheduled task."

I really want to use this...hopefully there will be a quick point update fixing these issues.  
(Version 3.0)