Hi Rain,
You bet you can purchase the
Passe Navigo Découverte as a tourist. Be sure to check out the "
Who can buy the Passe Navigo Découverte" part of the
Passe Navigo/Carte Orange article.
Walking between
Montparnasse train station and Montparnasse Bienvenue will take you about 10 to 15 minutes depending on how busy the station is (i.e. how many people are in your way). The walk is pretty easy. The two are connected via underground tunnels with a long moving sidewalk in between. Signage is very clear. Your only real decision is which Metro line you want to take from Montparnasse. There are 4 that connect to
Montparnasse/Montparnasse Bienvenue: Metro Lines 4, 6, 12, and 13. Check out the
Paris Metro Map or the
Paris Metro Trip Planner tool to help you figure out which one you should take.
Best regards,
Ben