You amp has no OUTputs for a sub, but if you really want to attach a sub you'll need to get one that is self-amplified and accepts >>>stereo<<< High Level or Speaker Level inputs.

Otherwise your amp's volume control won't also control the sub's volume.

You would feed it full-range audio from the speaker terminals on your amp and its internal amp would use that to develop its own power output for the sub.

Here's one example.

http://www.ch.onkyo.com/files.download/skw-205_manual_e.pdf