I see you are on Mac w/ OSX Yosemite.
There is a difference between texture files (which are images, like JPG, PNG, etc.) and SketchUp SKM material archive files. (The SKM archive have the texture file within it.)
If your textures are image files, you’ll need to put them somewhere in your user path easy to browse to. Something like:
Then inside SketchUp you need to create a new material using each texture. These new materials will be “In Model” only materials, until you save them out to a Custom Collection.
Since SketchUp 2014, more and more user files are being moved into the user path, rather than the application’s binary path.
I do not have a Mac, and the Materials Manager interface differs between the PC and Mac SketchUp editions.
Some of the Mac guys will need to weigh in here.
If they are already SKMs, put the sub-folder of SKMs someplace easy to find that is NOT SketchUp version dependant. Like:
And then within SketchUp create a new collection from the Materials browser context menu.
(Some Mac power users use symbolic links tricks to point at custom materials folders.)