Walk through the portal, pick up the Cube, and return to the main area of the Chamber.
Wait for the portal to show the Heavy Duty Super-Colliding Super Button, and then take the cube through the portal and place it on the button. Return to the main area.
Wait for the portal to show the exit door. Proceed through the portal and then through the exit door into the Chamberlock.
The radio in this level is located on top of a camera on the right side before dropping down into the pit. To gain progress on the Transmission Recieved achiement, you need to bring the radio to the exit door.
[ELEVATOR CHIME]
GLaDOS: Please place the Weighted Storage Cube on the 1500 Megawatt Aperture Science Heavy Duty Super Colliding Super Button.
GLaDOS: Perfect! Please move quickly to the Chamberlock as the effects of prolonged exposure to the button are not part of the test.