The Aix city bus line 150 connects the central bus station in Aix with ITER. One bus in the morning, one around noon, two buses back in the afternoon. Travel time is about one hour, fare is €1,20 one way, €2,20 return. The station next to Hôtel St.-Christophe is called "Rotonde Poste": left from the hotel, at the other side of the Apple Store, i.e. on the next street (without crossing it), in the direction towards the Rotonde.
ITER runs three commuter bus lines from/to Aix. (See all timetables and route maps.)
From Hôtel St.-Christophe, the ITER bus stop is left, to the other side of the Apple Store, left again, then ~200m to the beginning of the bridge over the railroad. They call it "Park Eurodif between Rotonde-Poste and Belges bus stops". The ITER buses (two lines) stop at the end of the "parking and bus street" that runs for a few hundred meters parallel to the street, about where the stairs toward the train station go down from the overpass level to the ground. (Look for "11 Avenue des Belges" on Google maps to see the traffic light, the top end of the stairs, and the area where the buses stop.)
Both buses leave there at 07:35h. One of the bus lines starts there, so the bus is usually parked a few minutes before. They carry large "Sumian" company labels on the side of the bus, and the direction indicators on the front say "A Aix Ouest #1" and "B Aix Ouest #2". One way fare is 2 EUR.
All buses' final stop is at the main entrance in front of the headquarter, where buses park next to each other in the bus parking area around 08:25h. (Some lines do an intermediate stop at one of the other gates first. Wait until everybody leaves.)
Following everyone onto the site, you will walk by the visitors center, where you have to report. Don't forget to bring your passport, else you will have trouble getting a badge. (I will pick you up from there.)
For the return trip, the buses leave at 17:40h from the main gate. (Dispatch from building 81 at 17:30h.)