I just put the pointy bit on the bottom dead centre into the plug wire, fold the two sides up as much as I can with my fingers. I then insert the plug wire into the rotor and push until it hits the bottom. This gives you a nice, tight fit. Make sure you put the waterproof cap on the wire before doing this and once the wire is in place, push the cap down over the rotor tower. I have never felt the need to buy a tool for this job when nature has provided me with fingers.
incidentally, you are showing the correct tool in your picture.