Thanks Aerilius, missed the “create a new material” link in your reply. Spent a few minutes reading the article. This clears up the meaning of the terms: material, color and texture.
Assuming the .skm files need to be in a folder as noted by [endlessfix].
This is the case, .skm files need to be in folders to appear in the list. Here is a post that outlines a few more of the Mac materials “quirks” that might save you some frustration.