In Settings » Add-ons (Demo URLs) don’t have a clue as to where to do the installation—what is “Install Add-on” looking to open?
The install add-on button is for browsing to an add-on that’s in a .zip.
The Demo URLs add-on is already installed. You just have to make sure it’s checked in Settings » Add-ons
Hmmm, my installation does not show a demo addon selection. NEVERMIND found it under enable Live.