I checked the download, everything should be fine. It's likely a folder structure issue. The zip file should be unzipped in your main assets folder, I've made it so the zip file has the appropriate folders and files as long as you don't change anything.
These work a bit differently than regular assets. The zip file needs to be unzipped in your main Wonderdraft folder, at the same level as assets, themes, etc.
They can be used as blending brushes at low opacity, or stamp brushes to make forests and other things.
Sorry for bothering you, but I'm having trouble trying to use one of your themes for Wonderdraft.
I've got the zip file in my folders, and I tried unzipping and just putting that in their alone. I'm using Mac btw, hope you can help? Thanks