You should just be able to run a subwoofer cable from the Pre-Out connection on the back of the receiver to one of the L or R RCA conncetions on the Subs Amp.
You'll either see a "LFE", "sub", or "subwoofer" preout/output on the back of your AVR (Audio Video Receiver) to the input on the back of your sub. Go into your AVR setup menu and set all speakers to the "small" setting (if I'm correct it should look like FS-CS-RS, since RadioShack based their AVRs on Pioneer models). Use a spl meter to calibrate and enjoy.