There is a staircase entrance to the bus station located on Atlantic Ave. South Station itself, an older "classic" architecture train station, attached to the newer bus station building, has an entrance on the next corner. Exiting the bus berths, you go down an escalator, and eventually down another, at the foot of which you can see the street. That's the Atlantic Ave entrance/exit. I note this because most people will be walking past it, then onto and past the commuter train platforms to get into South Station itself and the red line T subway station below it.