It shows up under the Plugins section of the OctoPrint settings.
Upon selecting it, it shows the title "System Commands Editor" at the top of the panel and then the rest of the panel is blank.
I tried it on a Mac with Safari, FireFox, and Chrome. The latter two showed the control box but clicking on it did nothing.
I see that this problem has been around for a couple of years in the GitHub docs.
Having just edited things back to normal, it now actually displays that single green input box. I think this is the author's actual attempt at an interface. Presumably, he expects you to right-mouse click the green rectangle which presents options "Create Command" and "Create Divider". Selecting the first then displays a new dialog box. I assume that things get smoother, having created the first one.