This is my Custom Content folder...
As you can see, I don't have these folders (Bitmap Brushes, Vector Brushes,) in my directory. I
think you might have some leftover folders from Moho 13.0.
In 13.5.5's default Custom Content folder, Brushes would hold any bitmap images you created for texturing vector strokes, and Scatter Brush would contain any Moho vector art used by the Scatter Brush tool. To see formatting examples for each, look at the files located here...
C:\Program Files\Moho\Moho 13.5.5\Resources\Support\Common\Brushes and
\Scatter Brushes
(Note #1: Your path may be slightly different from mine because I like to install each release of Moho in version specific folders. This allows me to run different versions of Moho on the same computer. If you don't do this, just drop '\Moho 13.5.5' from the pathname.)
(Note #2: DO NOT CHANGE ANYTHING IN THIS FOLDER. Changing files in the program's Resources folder is not recommended because it can potentially cause Moho to malfunction. Any file changes you wish to make to Moho should always be done in the Custom Content Folder.)
IMO, you should create a clean Custom Content folder for 13.5.x and build it up from there. To create a clean Custom Content folder, first quit Moho and rename your current Custom Content folder. When you relaunch Moho, it will ask you to select or create a new folder. Select the location where the 'old' folder was and Moho create a new folder there. Now you can update it by copying over any custom scripts or tools you may have added to the old folder.
Tip: Before you do that, make a duplicate of the default Custom Content folder and add '_BU' or '_default' at the end of its name. This will serve as a backup in case you mess up the folder your editing.
Hope this helps.