Hi Roger,
You can take the RER D in direction Melun / Malesherbes from
Gare du Nord to
Gare de Lyon and then transfer to Metro Line 14 direction Bibliothéque François Mitterand for one stop to Bercy station. First leg takes about 7 minutes. Second, about 1 minute. RER D trains leave Gare du Nord about every 4 - 12 minutes.
Ben