The plastic piece will pop itself into the hole. Like thief said, unbolt the pedal, then it is easier.
I have done this several times. Unhook the throttle body side of the cable, then once the pedal is unbolted you can pull it down to see where that plastic piece is. Get a small screw driver and pop the plastic piece out of the hole (It will come towards you) then once the plastic piece is out you will see that the cable portion (just the inner cable itself) will slide up and out of the pedal. There is an opening for it. When istalling the new cable just slide the pedal (pedal still unbolted) up into the metal cable part via the slot and pull back....you will see that the small white plastic piece will just pop right back in where it belongs. The last thing is where the cable goes through the firewall if I remember correctly it goes through and metal fingers pop out to hold it in. You need to release these from the inside of the car. This will all make sense once you are actually doing it.
Chris