To unlock Atton as a Jedi Sentinel, travel to the Refugee Sector on Nar Shadda. There, walk to the opposite side of the Refugee Sector until two Twileks approach you. They offer to sell you information about Atton. Purchase the information for 25 credits. Then ask Atton about what the Twileks told you. If your influence is high enough, you can train him to be a Jedi Sentinel (Light Side) or a Dark Jedi Sentinel (Dark Side).