Leo - thanks so much! I got it fixed, and learned some valuable techniques in the process.
For some reason when I pasted your XML code into my toolbar it didn't work - nothing happened. However, when I followed your link (How to use buttons and scripts from this forum) I was able to piece together how to get it to work. When I dragged the button to the desktop on a computer that still had the button, then copied that to the Surface Book, I could drag the button file onto the tool bar. I also opened up the button file and copied the XML, and pasted that into the toolbar, and that also worked. So perhaps there's a version difference between the version of DOpus I'm using and you're using?
In any case, I really appreciate the help, it not only got me over the problem I was having but added these great techniques to my DOpus toolbox. Many thanks!