the manual does not include a diagram showing all connections on the rear (and i'm not sure which of those covered models you actually have).
from what i can tell it appears that it accepts composite (yellow, red, white) connections from other sources. this being only stereo inputs.
some HTIB (home theater in a box) sets are known for only allowing 5.1 sound from the integrated dvd/blueray players. this may be one of those models however i can not say for sure without seeing the back panel of your unit.
if you would please find a photo (on google images is fine) of your back panel that would help confirm. basically what i am looking for is either a spdif optical or spdif coax connection. i did not see it listed in the manual so i dont think you have one but double check on the back.
as far as what you can do? not much. you might have a sound mode which extends sound to the other speakers (but its still stereo sound).