The mission to recruit Zaeed - Dossier: The Veteran - 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. Dock on Omega and simply head opposite from where the Normandy is docked to find Zaeed.
How to recruit Zaeed Massani
Cerberus has already made an agreement with the infamous mercenary Zaeed Massani, and with the matter of compensation settled, all you need to do is find him on Omega and talk to him. He’ll be busy roughing up a batarian he was hired to track down, which you can waste time questioning him about (but oddly, you can’t intervene in any way). Ultimately, however, accept his services and he’ll tell you about another condition - you have to help him complete another mission. This starts the mission Zaeed: The Price of Revenge.
After this Zaeed will conclude his business on Omega, and you’ll be able to deploy him as you see fit. If you want to hear some of his commentary from time to time, visit him on the Normandy, where you’ll find him in the now-unlocked Starboard Cargo Hold on the Engineering Deck of the Normandy.