The accessible entrance is on Via del Teatro di Marcello.
The building is owned by the commune of Rome. The ground floor is quite vast and is in the process (July 2015) of becoming an art/exhibition space. The second floor seems to be offices etc. There is a terrace area above, which is accessible by an ancient lift from the first floor. This lift, which was built in the early 20th century, is quite small but I got in with my Quantum Q6000 chair fairly easily. Once again I recommend a mid-wheel chair for manoeuvrability and lots of practice steering it. You don’t want to bang and scratch 100 year old lifts, after all. To get to the lift from the ground floor, there was a stair lift which only took a manual chair, so in 2009 or so I had to transfer into a dodgy manual chair with flat tires that they supplied and muddled along. Luckily now this lift is broken and a lovely Italian attendant took me on a very long, circuitous but interesting route right around the inside of the building which eventually delivered us to the lift, at the top of the stairs. We had a lovely talk in Italian about our respective families etc. At the top of the lift there is a very step ramp to the outside door. It must be nearly 30 degrees but my Q6000 was up to the task (just). If you’re nervous and alone, I suggest asking a fit looking fellow traveller to grab on behind for security. People usually love to help, I have always found. On the terrace, there is another high-tech external lift which takes you to the roof. They gave me the usual free ticket. Not sure if a companion has to pay; probably not, but if so, still a good deal, no? There are fantastic views from the roof in all directions, as well as well designed photographic signs with a legend to help you find notable landmarks. My elevating seat helped with seeing over the railing for photos, although the barriers are glass so you can see out OK even if you are low down. You also see the wonderful iconic bronzes of the winged horse ladies up close, and good views over the Forum. Once again my trusty elevating seat helped. The terrace level also has a cafe/restaurant which I did not try.