When you exit the hotel lobby, immediately in fromt of you will be escalators leading to several lower levels. This is where the RER trains leave from. On one of the lower levels is a ticket office where you can (if desired) buy one of the multi-zone passes (Paris Visite) for one or more days. Because these allow unlimited transfers on both the RER (regional commuter trains) and the Metro (city subway trains), they can be real bargains. The site below gives lots of additional information:

http://ratp.fr/

(Once you're on the English page, click on the PARIS VISITE link under the RATP FOR EVERYONE column for prices and information)