The subwoofer connects to the "mix audio out" near the speaker terminals. You'll have to set the crossover on the subwoofer to match the speakers low frequency extension. What speakers do you have?
Don't buy the stupid monster cable again, it's a waste of money. Any shielded 75 ohm RCA cable will work. This means most any audio or video RCA cable will work fine as a subwoofer cable, as long as it is long enough.