Make your way through the corridor, then drop down from the ledge when you reach it. The computer asks you to put the weighted storage cube onto the big red button. You will notice, if you wait for a few moments, that there is an orange portal in the room with you, and a blue portal appears in the different rooms around you that change positions every few seconds. Since you need the weighted storage cube, wait for the blue portal to appear in the room with the cube. Grab the cube and go back through the blue portal when it appears.
Back in the room with the orange portal, wait for the blue portal to appear in the room with the red button, then while holding the cube, go through and put the cube on the button. The computer will congratulate you, but will warn you that prolonged exposure to the red button is dangerous so you should hurry to the chamber lock. That sounds bogus to me, but whatever. Return to the room with the orange portal, then wait for the blue portal to move to the room with the chamber lock, and go through the orange portal then.
Go through the chamber lock and enter the elevator to the next level.