Afraid to say it but it might be more trouble than it's worth to get the subwoofer than you have bought working.
The subwoofer you've got comes as part of an upgraded stereo option extra for the stilo - I believe it's provided by BOSE and is a very, very similar system to that found on the Alfa 147 / GT.
It's part of a complete setup that works with the factory head unit and is supposed to include all eight door speakers (a mid and a tweet in each unit) and the subwoofer.
If you've got the factory AMP that goes with it it'll have a massive multi-pin plug on it I would guess. That's because it's supposed to have an in-put pair and output pair of wires for every door panel, plus power, ground, remote and singal wire and subwoofer output.
What's worse is I believe the BOSE speakers are something like 3 ohm impedance, as opposed to your normal 4 ohm door speakers - so the set is very much designed to work together as one.
If none of this sounds like and doesn't look like what you've got, please post up pictures of the sub / amp and connections it's got on it and we'll see if we can't figure it out.
If it is like I think.... I would suggest that if you've got the kit to do so, you measure the impedance of the subwoofer speaker you have and find out, unless you can see it stamped on it somewhere (usually on the base of the magnet at the back). Then try and find a small aftermarket amplifier that'll handle the speaker rating you have - it'll only need to be a small one. Probably then scrap the factory amp and fit the aftermarket amp to it.
