Terminal 1 appears on one of the monitors in the cockpit area of the ship(where Commander Keyes is). After going through the Calibration and after you have met with Commander Keyes, as you turn around to leave, there should be a computer with four monitors, each saying 'INCOMING MESSAGE'. Walk up to the computer and interact with it.
The terminal will activate a cinematic in which 343 Guilty Spark sends a warning message to the Pillar of Autumn, and then invites them to land on Halo and to make contact with them. The achievement activates once the cinematic is over.