Once arrived to the Termini station or the Ostiense station take the metro line B (blue) in the direction Laurentina. Get off at "San Paolo". Then take the bus number 23 in the Direction of Largo S. Leonardo Murialdo. The final stop is just in front of the Department.
Infos Ferrovie dello Stato: tel. +39 06892021
See http://www.adr.it (Airport Roma)