From the Marseilles downtown to Luminy Congress centre:
Travel to the "Rond Point du Prado" subway station (line 2, direction "Sainte Marguerite. Dromel")
then transfer to the 21 bus which will take you directly in front of the Luminy Congress Centre. (cost: 1.70 €)
(Buses circulate every 10 minutes between 5h15 and 21h weekdays, are very rare on sunday and inexistant during evenings)
Approximate journey length: 35 minutes