Hey. I may have figured out the flat materials issue (maybe). I need to know what version of Rhino you have (5, 6, or 7) and I need to know if the version of the library you installed has a 5 or a 7 on the end of its name. It's difficult to figure out what's happening on other people's computers, but I think with that info I *might* be able to at least confirm the issue (no guarantees I can fix it, tho).
The software is Rhino3D (version 7). And it has a built-in helper program called Grasshopper, which is a visual scripting program that can be used to automate some tasks in Rhino. Grasshopper used to be a plugin you had to install separately, now it comes pre-installed in Rhino since Rhino version 6. In this video, I am demonstrating a grasshopper script I made to make tapered gems.