It is called Podere Belvedere, but I just doublechecked on Google Maps and it seems to be that the waypoint that was on the map has been moved. It isn't right outside San Quirico d'Orcia. I don't recall the exact point now, I will have to do some research.
The old farmhouse is southeast, below what you see labeled as Podere Inferno. It is a private home and thus not labeled at all. The waypoint I suppose is close enough to where people set up to take photos, but I see many are actually in an olive grove which I think is closer to Podere Inferno as labeled on the map.
All these are private properties, so you can't just go into them with no regard -- I am sure they are used to people stopping by to take photos, just make sure you show the proper deference for the property by leaving no trash and destroying nothing. If you were to show up at the magic hour, I am sure you would find others there ready to snap some photos and thus make it easier for you to identify where you're supposed to go ;-).