In the link provided, it suggest searching for 'Materials', but that isn't the best choice...

SU should have two Material folders but only makes the 'User' one when you make a new material inside SU...

so it better to look for a redundent [still included] folder that has always been placed in the User path...

to open it in Finder you can copy/paste into Ruby Console...

on mine that's /Users/johns_iMac/Library/Application Support/SketchUp 2016/SketchUp/OldColors

In that SketchUp folder add a new folder named Materials [it will not get deleted on upgrading]...

add any material sub-folders into there, they must contain skm files to work...

once made drag it into the Finder sidebar for easy access
bit like this...

there are other gif mini tuts about add materials from inside SU...

john