A lock terminal is in room A31 (bottom left of the map. If you don't have the map, go back to the start of the game and go to the right nonstop until you're in a room you can keep going down staircases in). I destroyed it with the Bomb item (Y) that I got off the first boss. It is a large green computer (same as achievement picture) all the way on the left of the room and unlocks 3 different rooms.
EDIT:
Zedarboy confirms he got his in A26 without ever entering A33.