If you're collecting your tickets during race week, you'll need to go to the 'Rotonde' (the circular building beyond Parking Expo).
Getting there: from the main circuit entrance turn right past the museum and under the bridge. Take the sharp left hand turn just after the bridge and follow this road for a few hundred metres (Parking Expo is on your right) and the 'Rotonde' will be on your right hand side.
The 'Rotonde' is clearly marked on this map (just to the left of Parking Expo).