The Dossier: The Master Thief was originally added as DLC in the original version of Mass Effect 2. In the Legendary Edition, you can access it after completing the mission Freedom’s Progress.
How to get Kasumi Goto
To unlock the quest, you’ll need to find Kasumi’s terminal and talk with her. The terminal can be found at the Level 27 Dock in the Citadel, leave the Normandy, and the terminal can be found just outside it. Kasumi will ask you for the password which is “Silence is golden”, you can tell her to “cut the crap”, and you won’t be penalized for it. Go through the various dialogue options, and she’ll explain her reasoning for joining Cerberus, and what happened at her last heist, which is what she needs you help for. Once you have fully exhausted her dialogue, welcome her aboard the Normandy. You’ll now find her in the Port Observation Lounge on the crew deck of the Normandy.