The tab is located under the Anti-ventilation (horizontal) plate which site right atop the prop to prevent the prop from sucking surface water when turning. On smaller engines, designers assumed that propeller torque wouldn't be enough to cause handling problems so it was omitted. If your engine is designed to have one, on the underside of this plate, at the rear would be some sign of something being attached...for engines without one, the under side is flat and smooth.
Answer the question and once answered and installed if pending, you turn that tab in the direction the boat wants to go....standing behind the engine, if the boat wants to go to Starboard (right) turn the back end of the tab to Starboard) It only has so much effect and effect is dependent on engine tilt pin position ....but on a 15 hp, with the engine near vertical when referenced to the hull being horizontal, shouldn't be a problem.
Access is on top of the midsection just above the center of the tab. Usually a plastic cap is to be removed, exposing a Hex-socket bolt. You have to back the bolt out enough to get the tab over the indexing marks in some castings which aid in keeping it where you put it.
