Your cable box should have either a digital coax or an optical digital output. Yout receiver should have at least one of these inputs. Purchase either the digital coax cable or the optical cable and plug them in to the cable box and your receiver. That is the only way you will get 5.1 sound.
I was wondering how to incorprate my surround sound system while watching digital cable. I have digital cable using a cable box. On the manual, it has a very complicated diagram of how to hook up surround sound with my cable box and tv. It has all the complicated names and images. I am certainly not technical savvy whatsoever. What connectors do you generally need and how do you watch cable tv with surrond sound?