After you place two cubes on two red buttons, you reach Test Chamber 06.
High Energy Pellet
There is a High Energy Pellet bouncing up and down between a portal and a machine. There is also a pellet containment machine on the floor. Notice that the containment machine is shining an orange light on the ceiling. Place a blue portal there, and the pellet will go through the orange portal and into the containment machine. This will lower a translucent platform. Step onto it to be raised up to the elevator to Test Chamber 07.