Using a #3 posidrive or phillips screwdriver, unscrew your forward pressure screw until you can slide your heel piece off the back of the rail. Don't confuse this with the screw for adjusting DIN.
Make sure that the plate is securely screwed in to your ski. If it isn't then bring it to a shop. You shouldn't simply tighten the screws yourself as it will break the glue's seal. New glue should be put in everytime the binding screws are tightened.
If the plate is fine, then put your heel piece back in. Adjust the forward pressure by putting your boot into the binding. If the screw head is sticking out the back, then screw it in until it's flush with the body of the binding. It has to be flush at the bottom of the little cutout, not the face of the binding body. Check it a few times by removing your boot and putting it back into the binding. After a few times, the forward pressure screw should still be at the same position, flush with the bottom of the cutout.
That should be it!