This is a missable achievement.
There are three side missions total, you look for items while working on the main storyline. Play through the DLC as usual until the fighting stops and Shepard, Nyreen and Aria reach a command center full of her people. There will be a human girl, a Batarian and an Elcor. The Elcor appears later so talk to the girl and Batarian to trigger the missions and then feel free to go through the West Exit to continue the campaign
Omega: Assist The Hacker
A human hacker girl will ask you to enter a code into three terminals. If you are very thorough you can't miss them. The first one is right after you climb your first ladder, go inside the room to the right. The second one is after you go down some stairs and fight the first group of Cerberus, there will be a big symbol on the wall nearby. The third is after you fight your second group of Cerberus, there will be a Turian you need to heal sitting against the terminal.
Omega: Assist The Mechanic
A Batarian mechanic will ask you for a Kheri inverter. After you do the long Door Bypass and you fight a lot of Cerberus. Inside Nyreen's base by some crates, right next to stairs with the big Talon symbol and two guards. Give it to the Batarian after the loading screen takes you back to Aria's command center.
Omega: Assist Harrot
An Elcor named Harrot will ask you to send him coordinates if you run into Aria's couch. At some point you will reach an area with water flowing in the middle, you will have to turn on the dam controls and cross the water to proceed further. (You can't finish the DLC without doing this anyway so you can't miss it) Before proceeding further, climb down the ladder where the water is to find Aria's couch.
Edit: The achievement unlocks as soon as you find Aria's couch. Might be glitched since someone reported not unlocking it after doing everything right. Only the Kheri Inverter has to be turned in, the other two missions are ended automatically.