The drug den is the newest added area so I am going to assume you mean that. To acces the drug den, you must first complete the initial sections of Eliza Quinn's questline, which involves completing tasks given to you by the task board.
After that, you will become a lab assistant and be given the task of beating up certain inmates to obtain illegal pills from them.
After returning a few of these pills, you will be given access to the drug den at the bottom end of the main floor.