Adding/Creating materials in Rhino for Mac


I hope someone can help me with this and I hope this make sense. I am wanting to add a material into my library. I have downloaded it from food4rhino and tried to add it into Rhino. I’ve added a screengrab of the material page.

When I get onto my previous/new models screen it shows the preview as having the Black Walnut finish to the block. (see image, its really small!) but when I open it and render it, it can’t seem to find anything about it and the RMTL just shows up as black!

Can someone help me to add this into my library and get what I need? I can attach the files if needed
Thanks in advance.

Posting the file that is giving you trouble will help.