The mission to recruit Kasumi - Dossier: The Master Thief - was originally DLC in Mass Effect 2, but is included by default in the Legendary Edition. You’ll gain access to this mission after the events at Freedom’s Progress, after which you’ll find yourself in the Omega Nebula system. Simply travel to the Serpent Nebula cluster, where you’ll arrive in the Widow system, home of the Citadel.
How to recruit Kasumi Goto
Board the Citadel and when you arrive, immediately look to your right, where you’ll find an Advertisement pylon. Listen to it and give the correct password… or not. Either way you’ll find yourself talking to Kasumi, who will tell you about a heist to recover her partner’s “graybox”. Along with the money the Illusive Man paid her, your participation in this heist is part of the cost of doing business with Kasumi. This starts the mission [Kasumi: Stealing Money].
If you want to chat with Kasumi, you can now find her in the Port Observation Lounge on the crew deck of the Normandy.