The Orlybus takes you from Orly Sud or Ouest to "Porte d'Orléans" and "Place Denfert Rochereau". Departure approximately every 15 minutes. From either stop, take the metro line 4 in the direction of "Porte de Clignancourt" and get off at "St Placide".
The Orlyval shuttle takes you from Orly Sud to the RER B (Antony station), from there you can go to: Denfert Rocherau (12 min), Châtelet-Les Halles (15 min), Charles de Gaulle-Etoile (25 min), Gare du Nord (28 min). It runs from 6:00 am to 11:00 pm every day. There is a shuttle up to every 4 minutes at rush hour. In order to arrive at ICP, get off at Denfert-Rocherau and take the metro line 4, in the direction of "Porte de Clignancourt", and get off at the "St Placide" station.
Air France buses link the airports of Paris to different points of the city. From Orly airport take the one which goes to "Gare Montparnasse", from there you can go to ICP with the Metro line 12 to "Porte de la Chapelle", and get off at "Rennes", or line 4 direction "Porte de Clignancourt" and get off at "St Placide". You can also walk: from the Montparnasse tower, walk down "Rue de Rennes" and when you cross "Rue d'Assas", turn right (15 min walking from Gare Montparnasse). The Air France buses from Orly also go to Roissy Charles de Gaulle Airport, Gare de Lyon, place de L'Etoile and place des Invalides.
From Roissy - Charles de Gaulle
RER line B, take any train, they all make the central Paris stops: "Gare du Nord" (25 min), "Châtelet-Les-Halles" (28 min), "Denfert-Rochereau" (45 min). Get off at "Châtelet les Halles". Then, take metro line 4 direction "Porte de Clignancourt" and get off at "St Placide".
Roissybus, this bus takes you from Terminal 1, 2, 3 to "Opéra". Departure approximately every 15 minutes. Get off at "Opéra (rue du Scribe)" and take bus n°94 or n°95, get off at "Rennes". Please note: buses only stop if requested, you need to press the red button.
Air France buses link the airport and several places in Paris. Take the one which goes to "Gare Montparnasse", from there take either the Metro line 12 direction "Porte de la Chapelle", and get off at "Rennes", or the line 4 direction "Porte de Clignancourt" and get off at "St Placide". You can also walk : from the Montparnasse tower, walk down "Rue de Rennes" and when you cross "Rue d'Assas", take it on the right (15 min walking from Gare Montparnasse). From Roissy - Charles de Gaulle, Air France buses also goes to Gare de Lyon, Orly Airport, Place de l'Etoile and Porte Maillot.
From train stations:
Gare d'Austerlitz, take metro line 10 direction "Boulogne" and get off at "Sèvres-Babylone"
Gare de Lyon, RER line A direction "La Défense", and get off at "Châtelet les Halles". Then, take metro line 4 direction "Porte de Clignancourt" and get off at "St Placide".
Gare du Nord or Gare de l'Est, take metro line 4 direction "Porte de Clignancourt" and get off at "St Placide".
Gare Saint Lazare, take metro line 12 direction "Mairie d’Issy" and get off at "Rennes".
Gare Montparnasse, take the Metro line 12 to "Porte de la Chapelle", and get off at "Rennes" or the line 4 direction "Porte de Clignancourt" and get off at "St Placide". You can also walk: go behind the Montparnasse tower, then take "Rue de Rennes" and when you cross "Rue d'Assas", take it on the right (15 min walking from Gare Montparnasse).