CDG and Orly:
http://www.aeroportsdeparis.fr/
Paris Transit Network
http://www.ratp.fr/plan-interactif/carteidf.php?lang=uk
Paris RER/Metro route planner - SNCF Transilien
http://www.transilien.com/itineraire/rechercheitineraire/init
Air France Shuttle Bus with service from CDG to Orly airport (#3) and to Gare Montparnasse (#4)
http://en.lescarsairfrance.com/
There are two transit options from CDG airport to Gare Montparnasse:
a. RER Line B (destination B2 Robinson or B4 Saint Remy les Chevreuse) from CDG airport to Gare du Nord. Paris Metro Line 4 from Gare du Nord to Metro Station Montparnasse Bienvenue. Allow 1:00 hour. Allow an additional 15 minutes to walk from the Metro station to Gare Montparnasse.
~Note there are 3 points where it is possible to transfer from RER B to Metro line 4. A transfer at St Michel may be preferable as Gare du Nord is a large station and will require orientation time.
b. Air France operates a bus service from CDG direct to Gare Montparnasse. Allow 1:15 hours.