Alas, this is one of the ways the Materials Window on Mac is deficient. It can create a new texture from an image, but it can’t import or install an existing skm file! So far as I know (and if another Mac expert knows better, please correct me!) here’s what you have to do to install materials manually:
Before starting, one must realize that the Materials system on Mac does not understand hierarchical folders of materials. It wants all custom “lists” of materials to be single-level folders within your Materials folder. So you will need to flatten out the hierarchical folders from the Techo-Bloc folks after unzipping their SketchUpTextures.zip file.
In the Materials Window (opened via Window->Materials but confusingly labeled “Colors” in its top stripe), midway down the window there is a selector labelled “List”. Choose “New” from this list. In the dialog that pops up, choose a name for your new list (aka collection). For example, use one of the leaf folder names in the Techo-Bloc hierarchy.
Using Finder, navigate to your Materials folder. Unless you have customized the Files preference, the Materials folder is at
~/Library/Application Support/SketchUp 2017/SketchUp/Materials
Note that this is your personal ~/Library folder within your home folder, not the system’s /LIbrary folder. Finder hides your personal Library by default, so unless you have previously unhidden it you will need to use Finder’s Go->Go To Folder menu to open your Library.
- In your Materials folder you should see a subfolder with the name you entered in step 1. Copy the desired materials .skm files into that subfolder.
Repeat these steps for any additional lists you want to create.
- Restart SketchUp (it will not notice the new Materials until you do this). The selector at the top of the Materials window should now show an item for each of the lists you created.