The computer warns you that being able to create two portals will cause this test to take a very, VERY long time. That can't be true, can it? Anyway, make a portal on the ledge near the cube, and another portal near the floor that you can walk through. Put the cube on the button and go through the portal again to get through the chamber lock.
Drop down. You will want to make a portal on the wall above the pellet containment thing, and then a portal of the other color on the wall where the pellet is hitting it. This will start the unstationary scaffold, which has a weighted cube on it.
Create a portal on the ceiling above where the scaffold goes. Try looking straight up and walking on the floor under the path of the scaffold to ensure that the ceiling portal will drop you onto the scaffold. Make a portal of the other color on a wall and watch it, falling through when the scaffold passes underneath. Take the cube hop down onto the floor with it. Then create a portal above the higher-up platform that has a red button on it (there is a tile with one dot on it on that part of the ceiling). Once you have placed the portal over that platform, you can create a portal on the wall and drop the cube into it to make it land on the red button.
Once the cube is on the button on the taller platform, create a portal above the shorter platform (with the tile above it that has two dots). You need to fall onto that platform, so make a portal to go through and land on that second button. The chamber lock will open, but you can't go through it if you step off of the red button. The solution is to make a portal on the wall beyond that chamber lock, then make a portal of the other color in this room so that you can get off of the red button and go into the room beyond the chamber lock.
As you head for the elevator, the computer says that she can no longer lie to you, and claims that after the testing is over, you will be 'missed'. Being 'missed' is the kind of thing that people say about someone who died, so that doesn't sound too reassuring, does it? Anyway, get into the elevator.