The France/Benelux "extras" includes RER from CDG to Gard Nord. Does it use a flex day of your pass?
A railpass is good on RER lines which in whole or in part are operated by SNCF, the French national rail system. That includes the RER B line between CDG and Gare du Nord (but not beyond Gare du Nord). Yes, you would use a flex day of your pass. A single ticket from CDG to central Paris which includes transfers to other RER lines and/or Metro lines costs 8.20€ and will get you to within a few blocks of any hotel in the city. Unless you will be taking a train well beyond Paris the same day you arrive at CDG, don't waste a pass day.