The entrance to Pocket D in Atlas is to the East of City Hall, in the parking lot across from the Auction House and near the Architect Entertainment building.
After reviewing Atlas Park as the replacement for Founder's Falls, I believe it is appropriate given the zones connection to high level zones (Recluse's Victory, Rikti War Zone) and transportation hubs (Paragon City Monorail, TUNNEL connection, base portal, Fort Trident). If I wanted to connect to Founder's Falls I could use one of the many options available to a player to do so.