At the beginning of The Assault on Afterlife segment of the mission Omega: Aria T'Loak, defeat the Cerberus forces in the water treatment facility. When the bridge across the water channel is destroyed, raise the dam and cross it to the other side. Immediately on your right, there should be an access ladder on the side of the catwalk that descends into the part of the channel that was just drained. Climb down, and beneath the destroyed bridge are the remains of Aria's couch in a pile. Select the couch to complete the mission.