I can't find brushes in .blend files New blenderer here,
I have 2.9.1 and have downloaded https://blendswap.com/blend/20195, a free set of rock brushes.  When I append the .blend file, it does not contain a brushes folder (it does have freestylelinestyle, image, light, material, mesh, object, scene, texture and world).  I know there is supposed to be a brushes folder because when I first downloaded it, there was one.
I have downloaded the file again (twice) and reinstalled blender, but it can't find brush folders now in anything I download now, even different free brush sets.
And suggestions?
EDIT:  just to close the loop on this in case someone else has the problem, I had to reinstall 2.8.2.  The brushes folder would not appear in 2.8.3 or any version of 2.9
Maybe the version 2.9.1 isn't compatible with the version of the add-on, btw, 2.9.1 is not a release yet, is an Alpha version, and from my experience, in the alpha version there is not everything inside Blender activated or working.
Solution (Maybe): Search for another version of the brushes add-on or try with 2.9 of blender and go backward into the versions until you get the right one (because from the link you posted, on the left side, the specifications and details, show that is compatible from 2.7X (but keep an add-on up to date in Alpha version is going to be almost impossible
Hope this helps