After you retrieve a weighted storage cube from a pit to place on the red button, you reach Test Chamber 05.
Use Two Cubes
There are two red buttons here. You see a weighted storage cube on a platform, but you'll need a second cube to open the chamber lock. The second cube is in the pit wall across from the chamber lock. Create a blue portal under the cube in the pit to make it fall out of the orange portal. Then make a blue portal on a wall on your level to retrieve that cube and put it on a button.
Finally, go through the blue portal so you end up on the platform where the orange portal is. Then look at the platform that has the other cube and create a blue portal above that platform. Go through the orange portal to reach that cube. Put it on the other red button and go through the chamber lock. It will close behind you.
If you look around, you will see that there is an orange portal in the room above you. Make a blue portal anywhere, then go through it to reach the elevator to Test Chamber 06.