What OS? What version Inkscape? If you have previous version installed of Inkscape and haven’t deleted it and you are using Windows 10, you might double check that you are putting the extension files in the right program location. Long shot but I had that issue.
The above linked topic links to the extension I prefer for Inkscape to keep everything in one program and screen although the festi boxes really work. Get some cardboard and practice on them.
I really need to get that video made. I have one almost ready for making a Kleenex box cover. Here is the topic I have with a good illustration and file to use.