Ear mites can live fairly deep in the ear canal, or nearer to the surface, but regardless it's painful, and treatment isn't very nice either. If that is the issue, it needs to be resolved - I would get the vet out to see if that is the problem.
As for the clicker training, you should be able to stop by a local pet store and buy one, it's just a handheld device that clicks, a lot of them look like this: http://www.training-dogs.com/image-f...s-iclicker.jpg
And here is a very informative website on what clicker training is: http://www.canadianclickercentre.com...20Training.htm
In addition, you can buy clicker training books.
The basic idea behind it is:
1) establish a "bridge" - that is, click and reward, so click your clicker and immediately give a treat, and repeat this over a few times.
2) use a target (can be the end of a whip or something similar) and ask the horse to "target" and when he touches the end of the whip, click and reward. This will just help establish that doing something good means the horse hears a click and gets a treat
3) touch the area around your horse's ear (as close as she will let you get) and click and reward.
Eventually she will establish that the click is a reward on its own, it means she did something right, so you can decrease the click to treat ratio, and just give a treat every 3 or 5 or 7 times you reward with the click. Don't set an interval, rather give treats (after a click) at random points, i.e. you reward with a click for 4 times, then click and treat, then you reward with a click for 9 times, the click and treat, then 3 clicks then treat, etc etc, don't set an interval.
The only reason I suggest this is because with something so sensitive you want to be able to reward the horse as soon as you touch the ear, not 30 seconds afterwards.
Hope this helped! Let me know if I can clear it up further.