Unzip the file grbl-master.zip that you downloaded.
(In the Arduino IDE) Sketch > Include Library > Add .ZIP Library...
Select the grbl subfolder of the grbl-master folder you unzipped.
When I go to Add .ZIP Library and try to add the grbl sub folder it will not select it. It is like it doesn't see it. If I click on grbl sub folder it just opens the folder.
I have uninstalled Ardunio IDE and reinstalled and get the same thing as above.
I also tried copying the grbl sub folder into the Ardunio libraries folder and this does not work either as it puts the grbl sub folder into Sketchbook and not Examples. I see it in Sketches and when I try to upload it gives me a compiling error. I do not understand what is going on.
That is what I have done and it doesn't show up anywhere. What is strange is other Library's are in that same "Library's" folder and they show up in Examples folder. Something weird going on with that grbl folder. No part of the IDE recognizes it.