We flew into MRS last month and it was pretty simple to catch one of the various shuttle buses - they were just lined up just a very short walk from the terminal exit and each route clearly marked. We were actually headed to Aix-en-Provence, but according to this web page - http://www.marseille.aeroport.fr/fra...train.jsp#plan - there is a dedicated navette (shuttle) direct from Quai No. 2 and it takes only 5 min to reach the Marseille Vitrolles Aeroport Train Station..

Where are you heading to within France? The Aix-en-Provence TGV station is actually only about a 5-10 min bus ride from MRS airport and would be another option.