When I teach hook kick, I start with the leg motion separate from the kick. Try this: stand on one leg and lift your other leg so that your bent knee is as high as your hip, then kick straight down at the floor, as if you were kick starting a motorcycle. Make sure you pull your toes up. The way your musculature works, as your leg hits full extension, it will pull back in a hooking motion. Then try the same motion with your kicking leg parallel to the floor. Once that motion works, try turning backward as you would for a back kick, with your leg up from the beginning of the turn. Turning back hook kicks are often easier than lead leg or rear leg turning forward because the motion puts your hip into the right position for the kick. Once you can do the kick turning backward, try turning forward.
Other tips: the motion of the kick comes from your hip, not your leg; the kick should cover about 12-15 inches with your leg extended and toes back before your leg bends again. The only way this will work is if you pull your entire leg from the hip. I understand what your instructor is saying - hook kick is great for hitting the back of the head - but I would suggest you get the basic kick down before you try to raise it. If you have access to a video camera, you might take some video of the kick so you can look at it and see exactly what your leg position is - that kind of feedback can be very useful when you're having trouble with a technique.
This may not be the way your instructor teaches or performs hook kick, so ignore it if it's not - but if it is, I hope it helps.