Drag this link, or add it by transcribing it, into the "Download add-ons from these servers" addon manager configuration (Options menu -> Preferences... item -> Install add-ons pane): http://xml2rfc-xxe.googlecode.com/svn/release/xml2rfc.xxe_addon . Then go to the addon manager (Options menu -> Install Addons...) and you will see the xml2rfc configuration in the list of addons to add. After getting this set up, you can do further upgrades, etc. of the addon using xxe's addon manager.