First we need the gauss; you can't get into the secret without it.
For those who don't know where it (the gauss) is, the screenshot shows it.
Grab the gauss and run to the wooden ladder. The secret is behind our portal's logo.
There's a small recess in the wall, so if you jump to the logo, you won't fall to the ground. Once you've done that, shoot the crate with the gauss's white beam.
Then the doors will open and you can enter the secret room
