I just received the Eurail Pass I ordered (Benelux-France). In their Traveler's Guide under "Extra Pass Benefits" for France, it says: "RER (Paris underground) from Paris Charles de Gaulle airport to Paris Gare du Nord railway station (in this direction only): free transportation. In order to obtain access to this RER train, go the the RER station at CDG Terminal 1. Ask for a 'contremarque' at the ticket window …", etc.
Does this use up a Flexi day? I was not planning any major train trips that day.