In the case of Painter 7 and newer brush files and folders, unless they are in exactly the right location, the corresponding variants will not load. The diagram above shows the correct folder and file hierarchy for a custom brush library (Ace Brushes in this example). The Ace Brushes library folder contains a brush category jpg icon image, and the associated brush category folder of the same name (Cool Crayons).

In this example, the brush category folder contains a captured dab type brush variant xml file (Grainy Wax.xml), and the associated captured dab jpg file of the same name. A second brush variant xml file (Smooth Crayon.xml) also resides in this folder, and is of the non-captured dab type.

Let's take a look at two out of the possible error messages associated with brush files and folders being in the wrong location when we attempt to load the brush library.
Although the library loaded, this message came up when I selected the Grainy Wax variant in the above example. It transpired that this was a captured dab type variant, and I had neglected to include the associated jpg file (Grainy Wax.jpg) in the Cool Crayons brush category folder.
In my eagerness to try these downloaded brushes, I placed the unzipped enclosing folded (which housed the brush library and a Readme document) directly inside the Brushes folder. Although the variants were somewhere inside, Painter could not find them. What I needed to do was to place only the brush library folder and the contents thereof, in the Brushes folder.

Sometimes, a brush library may be directly zipped and not include a Readme file or enclosing folder. In that case, my first course of action would have been correct.

Sandman's Note;

There are of course many possible variations on the above, but only one will give error free brush loading (at least from a location aspect). Taking a little time to check your brush file and folder locations are correct, before you attempt to load the library can save a lot of frustration later.

If single xml brush variant files are downloaded (with an accompanying jpg file in the case of captured dab type variants), either place the files in an existing brush category folder, or create a new category (and possibly a new brush library also) to house them.
Addressing Painter Brush File/Folder Location Errors