We strongly recommend that you book your accommodation as early as possible. Lausanne is very busy during the summer and finding accommodation can be difficult.
Hotels have reserved rooms for IUSSI participants, you can book them here. You can also find information about hotels to stay in Lausanne on the tourism office website. For smaller budgets, there is the Lausanne Youth Hostel Jeunotel, as well as the Camping de Vidy which offers 2 or 4 person bungalows very close to the conference venue.
Note that staying in a hotel, hostel or at a campsite in Lausanne normally entitles you to a Lausanne Transport Card which allows you to use public transport for free and offers other benefits. You can find out more details in the tourism office website.
A detailed map of the city is available here.