Found the icon again. Rated 5 out of 5 stars
I have the navigation toolbar hidden. The ruler icon found its way to the navigation toolbar. Moved it back to the Add-on bar (where it should be by default!) and it's usable and visible again.This review is for a previous version of the add-on (0.4.13.1-signed). This user has 2 previous reviews of this add-on.