CHES 2015 will take place in the Palais du Grand Large, congress center of Saint Malo (a view of inside is available here). It is located close to the old city of Saint Malo (Saint-Malo Intra Muros), which is the ideal place for your stay. See the following map.
You can book your hotel using the CHES 2015 booking form: http://www.pepss.com/gti/0151/GTI_151_STMALO/221/
For information about tourism in Saint-Malo area see the tourist office website.