here is my Solution for this Achievement. Its possible after the DNA Sequence 5-4. (Rescue Terry)
You have a new Symbol on the Davenport Map, go to this Symbol and help the Person (kill all Targets), after this and a little Cutscene the Person (Lance) settled in Davenport
PS: More AC3 Videoguides found you here - http://www.youtube.com/playlist?list=PLuGZAFj5iqHcNAlGL214X0...