Follow the lines from the numbers to the letters.
Jump through the evil gears on the left.
Hint:
You unlocked it once. Try doing it again.
Solution:
You only get one chance. If you fail, you must unlock it again. Find the computer that unlocks Terminal 1. It is located at the end of the tunnel with lots of yellow liquid falling from the ceiling.
If you really give up, here is a video:
http://www.youtube.com/watch?v=zrZLg1moewo#t=1m15s