The building that has Nora is in the NW part of the blue circle on the map, just to the east of the bridge that leads to the Industrial District. It is also the same building as where you can switch to Catwoman. From the bridge, throw a freeze grenade into the water for a raft and you should see an anchor you can use to draw you closer to the destructible wall. Blow the wall and pull the raft into the building. Hop up onto the ledge and go through the door there.
Climb into the vent and into the next room. This is the only other room in the building and it's a fight. There are ten enemies in here, two have armor, two have shields, and two have knives. There are also plenty of things to be thrown and two weapon chests. In this fight, you need to be very aggressive with your takedowns and baterangs. Make sure you keep moving and once you've cleared the room, check out Nora. You'll be told to return to Mr. Freeze in the GCPD. Do so to complete the mission.
