You can plan your travel route easily by yourself. You can find the "centre de physique théorique" on google maps. Still you can find some more detail below:

By train:

There are various trains arriving at the Marseille central train station "Marseille Gare St. Charles". From there the campus can be reached via the métro M1 or M2. In the train station follow the signs down to the métro. Tickets can be bought at the machine before the entrace (there are one way ticket, a 3 day pass, or 10 rides tickets).

Take then either the métro M1 to "Castellane" or M2 to "Rond Point du Prado" (direction Sainte-Marguerite Dromel). In both cases leave the métro station and take the bus B1 in direction "PN des Calanques"/"Campus de Luminy". There is a big street going straight to the south of Marseille. The bus stations are at this street. You can get off the bus at either "Luminy PN des Calanquess" or "Luminy Faculté".

By plane:

You can arrive in Marseille with the plane at the Aéroport Marseille Provence (MRS).

When you exit the terminal, between Terminal 1 and 2 (see map on the left), there is a big bus station. You will have to take the bus L91 in direction Marseille St. Charles. You will find there also a ticket machine, where you can buy a one way ticket for L91 for 10€. The bus only stops at Gare St. Charles. From there you can follow the description above.

Alternatively, you can get to the train station "Gare de Vitrolles (VAMP)" using the bus 13 or by walk (20 min) and continue with a train to Marseille St. Charles.

In general, there is the app "RTM Régie des Transports Métropolitans", which is useful to plan any travel across the city using public transport. Else also google does a good job in finding relevant buses etc.