Head to Central City and clear out the stage near the museum, and then loot the GPS tracker from the secret service agent corpse on stage.
After leaving Central City make your way to the Mayor's mansion in NE Sunset Hills. Clear out all of the enemy soldiers as you fight to the back yard to get inside the mansion, then move upstairs to get the key to the presidents room off of a secret service agent near the door. Once you secure the president the DLC will end.
Note: Be sure to complete all other achievements (except Duty or Death, obviously) prior to talking to the President and ending the DLC.