Yes you have two knock sensors I believe, both at the back of the engine. Take the black plastic rubbish off the top of your engine and you will see a blue plug and a white plug on the left hand side on a mounting. One is for the crank position(phase) sensor, and the other is the top knock sensor. The other knock sensor is located lower down on the block, but you should be able to see it peering down the back of the engine with the black plastic binliner rubbish off. They are round and black with a bolt through the centre of them.
Whether your engine can utilise Optimax depends on whether the ECU can advance the ignition enough to cater for it. I don't know whether a Stilo ECU can or can't but you could figure it out very easily by plugging in an OBDII display and reading the advance whilst booting it in a given gear at given revs, then filling it with Optimax and doing the same. The only way to know whether you are definitively getting a performance gain is to do that, or put it on the rollers and find out.
