Terminal 1
The first time you go inside, you will see a door almost immediately on the left. Simply enter this area of the level to find the first of the seven terminals.
Terminal 2
When you go inside the structure to let the tanks across the bridge of light, there will be a Terminal almost directly behind the controls for the bridge.
Terminal 3
You’ll miss this terminal if you are not careful. After defeating the scarab, you will head into a large structure. There will be a room with several sleeping grunts. Nearly directly following this room is another room in which you can take two different directions. There is a door, and a staircase (both eventually lead to the next room).
Underneath the staircase is an additional door that leads to the 3rd terminal.
Terminals 4, 5, and 6 can be found on the 7th level, ‘The Covenant’.
Terminal 4
When you access the first Elevator leading to the tower controls, jump onto the platform to the left of the elevator control panel. The terminal is dead ahead.
Terminal 5
After getting a Hornet vehicle, fly towards tower 2 (the one that the Arbiter has already disabled). A voice will remind you that Tower 2 is already clear, just to confirm you are at the right place. The Terminal is just inside of the structure.
Terminal 6
Terminal 6 is identical to Terminal 4, only in the 3rd tower. Just jump to the platform, and there it is.
Terminal 7 can be found on the final level of the game, ‘Halo’.
Terminal 7
Terminal 7 is found at the beginning of the final level in Halo 3. To find it, simply head forward from the start of the level, until you encounter the area with a few structural beams going from one wall into another. At this point, turn right, and look for a beam separate from the rest. Follow this beam into a doorway. Cortana will ask you, “Where are you going?”. Turn left once inside, and the Terminal should be in full view.
Access them all and consider yourself ‘Marathon Man’.