The best way to get to Potentino is to fly to Rome Fiumicino Airport and then hire a car. (It seems the most difficult thing is finding the Auto Noleggio or Car Rentals, which is across the airbridge in the building behind the airport on first floor level!).
Then you drive north up the coastal autostrada towards Citiavecchia and Grosetto, veering right towards Castel del Piano in the hills back from the coast, and then Seggiano. The trip to Potentino from Rome Airport takes about 2.5 hours by car. Another hint, on leaving the airport, is to first take the road towards Rome, before branching off in a northerly direction.
It's also possible to fly into Florence Airport, which is a slightly shorter trip to the castle. However flights to here are not as frequent (and it's more than a little difficult navigating your way out of Florence!). Again, take directions towards Rome first, before taking the autostrada route towards Siena (remembering the old saying 'all roads lead to Rome'!)
It is also possible to take a bus from Rome airport, and then another to Castel del Piano. This takes about a day!