This quest tasks you with killing President Kimball, given to you by Caesar toward the end of the Give Unto Caesar questline.
Upon receiving the quest from Caesar, you are told to meet up with Cato, who can be found just west of Hoover Dam. Go to the waypoint on your map, speak with Cato and he will have you wait until the next day.
Upon waking up, speak with him and tell him you are ready. He will then give you a NCR disguise to put on, do so and follow him to the Dam. When you get to the Dam, avoid the Ranger patrols at all costs, as they will immediately see through you disguise and start shooting at you. Instead, sneak around them and speak to Cato in the crowd.
You will now have several options to perform the assassination:
50 Explosives - Speak with Cato about rigging a bomb on the Vertibird
50 Science - Shoot the aircraft down as it arrives, with Cato's help
50 Repair - Rig the AA gun to explode via the terminal just east of the Visitor Center door.
lobsterparty adds:
If you want to either use the 50 science or 50 repair you have to use the electronic box up the stairs.
Or you can always just shoot him in the head and make a break for it, likely to survive only on the easier difficulties however.
After the president is killed, run from the Hoover Damn until you are able to fast travel back to The Fort to talk to Caesar.
Credit to Chunder Dragon for pointing this out:
BE SURE TO TAKE OFF YOUR DISGUISE BEFORE ENTERING THE FORT!
Quest Complete!