There have been some past forum threads about visiting the Palazzo Farnese and I did some research as I wanted to visit in September. The info below gives info on contacting the company that gives the tours, Inventer Rome.
In order to open the Palazzo Farnese to a large audience, the French Embassy in Italy entrusted the Inventer Rome association with the organization of visits to the palace. The route of the visit allows you to discover the Sangallo atrium, the cortile, the garden, the salon of Hercules with its tapestries inspired by Raphael's frescoes and the gallery of Carracci. The visit is directly online, on the website of the association. Groups are invited to contact "Invent Rome" directly by e-mail. Visits are offered in French, Italian or English on Mondays, Wednesdays and Fridays afternoons and accompanied by a professional guide. A contribution of € 9 per visitor, to cover the costs of this service, is requested. For any question concerning the visits, we thank you to contact the association via its Internet site: http://inventerrome.com Note: The association Inventer Rome is the only one authorized by the embassy of France to organize the visits of the Farnese Palace