I find the sites where they are supposed to be, but are not in English, and or will not download
They run about 3 and hour; just go to the bus station near SMN and get a ticket and get the next bus. The return schedule is posted at the spot they let you out in central Siena.
I had the same issue. Do they run at all hours?
Download the timetables from here:
http://www.tiemmespa.it/index.php/Viaggia-con-noi/Orari-e-linee/Siena/Extraurbano
The bus number is
131 (slow)
or
131R (Rapid - via freeway)
A=andata (going) R=returning
FERIALI = weekdays (MON to SAT)
FESTIVI= Sundays and Holidays.
The numbers (for the hours and minutes) are the same in Italian and English, except for the use on the 24hr clock. If you have difficulty translating the 24hr clock, deduct 12 from the hours in the afternoon, or ask any friend or relative who is in the military or works in your local hospital.