To save a Material for future use it must be saved in a list inside the materials browser. Switch to a view of all the material lists by clicking on the brick icon in the materials browser. The stock lists and their materials, like “asphalt and concrete” Glass and Mirrors" are there, they are un-editable. To save custom materials for use in all SketchUp files you must first make a new list. From the list menu at the bottom chose new and you will be prompted to give your new list a name, this will make a new empty list which you can drag materials into from the current material swatch box in the lower left. Or, set the material browser to the “in model” materials and from the list menu below choose duplicate, you will be prompted to give your new list a name and the new list will have a copy of every material that was in the list which you duplicated. Note that lists are presented alphabetically and this is not controllable, to keep things organized I start all my custom lists with the letter z so they stay grouped at the bottom of the list.
There are Quirks on the mac interface: Colors cannot be removed from a list via the material browser, the remove option does not work, it’s a bug. Lists cannot be renamed or removed via the material browser, again this option is present in the menu system but it does not work. To manage the list names and delete lists or individual materials from an existing list it’s best to enter the file system and do this manually. The file pathways are available under Preferences>Files>Materials, hit the folder icon on the right.
Some more reading on this complex subject: