There is a screw on the back of the heel piece, on the bottom, near the topsheet of the ski. Turning the screw clockwise moves the heel forward, counterclockwise moves it back. Get it close and then click the boot in. That adjustment screw should become flush with the housing of the binding. If it is still sticking out, release the boot, turn the screw clockwise a few turns, then click in again. When you have it right, the screw will be flush or slightly recessed.
Don't turn the screw while the boot is clicked in.