You need a stereo 3.5mm to 2 RCA adapter... a stereo 3.5mm adapter has two black rings on the silver part.
More then likely, either you are using a mono 3.5mm adapter, or if you are using a stereo adapter you may be trying to plug the subwoofer into the center channel output.
Also, keep in mind that subwoofers need to powered. Either through a built-in amplifier (aka "powered subwoofer") or through a seperate amplifier.
Just my guesses...