I agree that information is a bit hard to find.
http://wiki.flightgear.org/Addons is one of the places to look for (it has a link to the repository). The file README.add-ons in $FGDATA/Docs is mostly for developers. If you are using the
http://wiki.flightgear.org/FlightGear_Qt_launcher then you can either use the command-line option at the bottom of "Settings" in "Additional Settings" or "Add-on Module Folders" in "Add-ons".
I have never used add-ons, so I actually do not know. In case it works for you, please consider updating one of the wiki-pages with your findings.