you either need to connect the red/white parts of the component lead, to the surround, and live with stereo, with rear echo(depending on what sound field you pick on the surround, like pro-logic), or you need to buy an optical - coax convertor, an optical lead, and a coax lead, to get 5.1 from your system with the 360..
the 360's output is optical, but your system has a coax in for AUX connections, thus the need for a convertor, and extra leads.
all in, you are looking at spending around £15 delivered :
optical - coax convertor optical audio lead coax audio cable