You should hook up the speakers from the Monitor Out L&R on the back of the MBox mini.
As for levels...
I would turn itunes up to full volume. I think its safer -there is no risk of a sudden accidental rise in level. Because this is the start of the chain a small change in volume could have a dangerous impact if you have your speakers running full blast.
Thats my personal preference.
There are a few schools of thought for using active monitors. The first suggests that you turn up their individual gains to maximum. This works for a lot of people.
But another way (and the one that I follow) is to check the manufacturer's instructions. Eg the gain control on my active monitors clicks into place at 12.00, and the instruction manual recommends this as the optimum level.
I figure the manufacturer knows a lot more about its monitors than I do. So maybe you should check out what MAudio has to say.
(And get into the habit of turning your speakers on last, and turning them off first.)
So once you've set you monitor levels, and set your internal levels, you can use the Mbox as your main volume control.
Hope that helps!