Okay, I remember this one.
1: Grab the block and set it on the leftmost button. The door on the left should open.
2: Move left and fire a portal up and down, up first.
3: Get back to the block, grab it, and drop it into the portal. The block should land on the closed trapdoor.
4: Move on top of the leftmost button. The trapdoor should open, letting the block fall into an infinite loop.
5: Fire a portal down left (using ). The block should fly up beside you.
6: Quickly, jump over the portal and fire another portal down. If you did it fast enough, the block should be jumping up and down perpetually.
7: Move underneath the rightmost trapdoor.
8: When the block is on the left side, quickly fire a portal down and move on top of the rightmost button.
9: The block should fly above the trap door. Quickly, when it's above the trapdoor, get off the button.
10: Now the block is in position. Portal yourself up, put the block on the button, and you're good!
I hope that helps - it works from my memory, but I haven't tested it. (C'mon Kerm, make Jstified better!)