After a your first skirmish with the Prometheans, you'll come to a large area with rocky terrain. Battle through that until you're standing on a small 'balcony'-like forerunner structure. Crawlers and a Knight will jump onto the balcony from the ground below. When the enemies are cleared, drop down. The progression of the level will have you continue up a dirt path to the right. To the left, in a forerunner alcove, is where the terminal is located.