It IS a bit tricky, but its how a local would do it. In CDG, follow the signs that say "Paris By Train" . Its a bit of a walk from terminals 2E and 2F, which is likely where you are coming from. Go down the escalator. Buy a ticket for the RER from a machine. It was 10 euro last time I did it. Find the RER-B platform, and get on the next train. You may need to ask someone where to stand, as its confusing for some. I would get off at station Port Royale, after about 40 minutes, and walk west on Montparnasse blvd. Its not even a kilometer to the station from there. There are hotels of all shapes and sizes near the station. I expect a google search will find better instructions than these..........