*POSSIBLE SPOILERS*
To get this, you must have bought either a new copy of the game or the Catwoman DLC, obviously ;D
Towards the end of the game, after completing the Steel Mill for the second time and the Ghost Train achievement, you will switch to the Catwoman storyline. Follow the objective markers until you get underground into a vault patrolled by TYGER guards. Climb on the ceiling and drop down to pickpocket the three ID cards, go back to the computer in the first room, then proceed to clear out the rest of the guards. Open the vault/safe, walk inside, and press A on the briefcases right in front of you.