Once you have downloaded the map, the option to view them (the offline map) appears on the bottom of the map page, where you should see three options: Offline, Standard, and Satellite.
Edited to add that to get to the map page, tap the map or compass icon in the upper right. The former will open to the location of the place you are viewing (a city or an accommodation), the latter opens to your physical location. In either case, once you are on the map page you can zoom to your location (again with the compass icon) or change the map types.
Edited again to add that the offline maps only cover the area immediately surrounding the camino, so if you are in Wisconsin and switch to offline maps, you won't be very impressed.