Place one end of a portal on the ceiling above the middle of the platform's track, and the other end of the portal on an accessible wall. Next, you need to get the cube off the platform, and onto the first button. The energy pellet will go into the receiver, and the platform will start moving. Place one end of a portal on the energy pellet's scorch mark on the wall, and the other where the receiver is projecting a red spot onto the ceiling. To begin with, the energy pellet needs directing into the receiver so that the platform holding this room's cube can start moving. Solving the main room requires the use of several different portals, so that two cubes can press the two buttons, opening the exit door so you can walk through.
