Help with turtle beach PX21?

So i have a pair of Turtle Beach PX21's for my ps3. i amm currently using the logitech z 5500 surround sound system. my ps3 is plugged in via HDMI to my tv, then from the tv is an optical cable to the surround sound. how can i use my PX21's?, do i plug them in via usb to the ps3, then plug the other lead into the headphone part of the decoder on my z5500?? please help, im so confused. : S