The Dossier: The Veteran 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 Zaeed Massani
Cerberus has already made an agreement with the infamous mercenary Zaeed Massani, and all that you need to do is pick him up at Omega. Upon docking there, and exiting the Normandy, you’ll find an oldish man brutalizing a prisoner at the end of the hallway. As you go over to him, you’ll have a few dialogue options regarding his attitude towards the prisoner. Welcome him aboard, and you’ll find him in the now unlocked Starboard Cargo Hold on the Engineering Deck of the Normandy.