The skinning system for this framework can be a pain sometimes. I downloaded it again and ran the samples. Getting started sample worked fine. But the ScrollBar sample threw the exception you are describing.
Something really needs to be done about why the skinning system can seem so hard to get working sometimes. Other then that, once you get it running it can be a great GUI system.
I've tried extracting the *.skin files into there own folders, and that does not help either, *.skin files are just zip files renamed. I had this same issue when I started working with this framework, and eventually got it working but it was so long
ago I can't recall what it was I did to get it running.
Keep trying and you should be able to get it working eventually.