Make sure Sub is set to ON on your reciever, set the crossover to say, 80hz as a starting point.
Plug a subwoofer cable (RCA to RCA) from SUB OUT or LFE OUT on your reciever to Input L (or R) on your sub. Turn the sub ON, crossover ALL THE WAY UP, and put the gain up about halfway. You're done.
This isn't rocket science guys. If you REALLY want, you can get a Y cable to plug into both the L and R inputs on the sub. Why are the guys here having such a difficult time? I don't mean to be condescending, it's just 1-2-3.