Hi rhonsberger,
Please refer to the examples below, it shows you how to add the custom button for the toolbar. You can find the full source code from the download package.
http://richtexteditor.com/demo/custom_buttons.aspx
http://richtexteditor.com/demo/custom_buttons2.aspx
http://richtexteditor.com/demo/custom_buttons3.aspx
Regards,
Ken