But ironically enough, you have to work for the Powder Gangers first. If you're already vilified by them, try wearing one of their suits as a disguise and approach the facility. Go into the administration office and talk to Eddie - he'll give you a bunch of quests to do. Eventually, he'll want you to find out if the NCR is going to attack the facility or not - which they are. At that point, you can either warn Eddie of the attack or betray Eddie by going to the NCR instead and asking if you can help take the prison back over.