Basically put as may raisers are that your feel safe and comfrotable with, try it out and see, and if you don't like it modifiy, As your adding put a few risiers on, put the top back on and do a few steps with it, and see if it's causing you any problems if not go higher. And if I recall correctly in her high step workout she actually tells you how high your step should be once you get to her level. I haven't used it in a while but I remember right it was between 15 - 17 inches.
Also if you have knee problems don't bend your knee any more then 90 degrees to step up. The more it goes less of 90 degress, (lifting your leg higher) then you are putting more stress on the knee.
If you got healthy knees you don't have to worry about that, but if you do notice any discomfort lower the step, anyway as your knee muscles may not be strong enough, at this time to lift your whole body over and over, more then a foot off the ground, this isn't usually something we do. Stairs are somewhere between 4 to 10 inches, curbs ae usually somewhere between that, so in daily life we just don't get much work on the knees to lift us higher, so don't give up if you can't hit the 15 inch right way, just do what you can and you'll get to where you want to be in no time.
Kit