Summary: RER line B train, get off at “Le Bourget” station (or Metro Line 7, get off at “Fort d’Aubervilliers” station) and take the free shuttle from there to the site.

From the COP21 FAQ:

How does one get to the Paris-Le Bourget site?

To travel from Paris to Le Bourget, you are strongly advised to use public transport serving the site.

You can take the northbound regional express railway (RER) line B train (in the direction of Charles de Gaulle Airport – Mitry Claye) and get off at the “Le Bourget” station. Exceptionally, throughout the COP21 Conference, all trains will stop at the Le Bourget station. A special shuttle bus will take you from there to the COP21 site.

You can also take Metro line 7 to the “Fort d’Aubervilliers” station. You can also take a special shuttle bus from there to the conference site.

The Paris-Le Bourget site can also be accessed by regular bus routes:

– bus 152 departing from Metro line 7 at the Porte de la Villette – get off at the “Parc des expositions du Bourget” stop;

– bus 350 departing from the Gare de l’Est, the Gare du Nord or the Porte de la Chapelle (Metro line 12) – get off at the “Parc des expositions du Bourget” stop;

There will be a special bus link operating 24 hours a day from the Roissy-Charles-de-Gaulle hotel zone to the conference site.

Lastly, several special bus links will run at night only from the conference site to the principal hotel zones in Paris.

The Paris-Le Bourget site can be accessed by car on the A86, A1 and A3 motorways, but you may not be able to park and the very dense traffic on these roads makes this a very unreliable method of transport.

via FAQs | COP21 – United Nations Conference on Climate Change.