This page explains the procedure for installing your own custom mapping symbols.
There is an example asset pack on your Humble Bundle download page. Starting with the example asset pack is a good idea as it has a pretty complete structure already with examples of different types of assets...
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