You may report your arrival at Diable Registration Centre:
3824 Chemin du Lac Supérieur
Lac Supérieur QC J0T 1P0
(819) 688-2281
46°15'41.5"N 74°30'35.6"W
46.261539, -74.509896
Alternatively you may go straight to Discovery Centre at the Lac Monroe campground:
1 Chemin du Lac Supérieur
Saint-Faustin-Lac-Carré QC J0T
(819) 688-2281
46°19'49.9"N 74°29'59.2"W
46.330538, -74.499770
The offices have different working hours, but complement each other in terms of meeting the campers.