If you don't have the manual for your sub, you can find it
here.
If you are using left and right preamp outs, simply run a pair of RCA cables. I'm guessing there isn't a separate subwoofer out but if there is, you can run a single RCA cable to one of the input jacks on the sub, usually the left one.
You'll probably also need to experiment with the crossover dial on the sub to achieve the best blending. With your setup, all frequencies are being sent to the speakers but the sub will be adding to the lower frequencies. That may take some trial and error to make it sound good.
Jim