You should have one weighted storage cube already, carried over from the first part of the testchamber. There are two red floor switches in the room, and both are located on elevated platforms on the left side of the room. The pellet should pass through the portals and enter the receptor on the ground. Now place the orange portal on top of the black pock mark that's made by the bouncing high energy pellet. You can spot a faint light on the ceiling that'll serve as a guide for where to place the blue portal. Fire a blue portal onto the ceiling, directly over the energy pellet receptor that's on the ground. There's a bouncing high energy pellet in the room that's worth taking care of now to avoid future problems.
Grab the storage cube and take it through the nearby orange portal to reach the next room of the testchamber with the weighted storage cube in hand. Fire a blue portal into the next room, anywhere, then drop back down to the weighted storage cube. Go back through the orange portal to return to the catwalk and look out the doorway you just opened (don't drop down yet!).
