Many women and new gym goers are intimidated by this machine: the assisted pull-up. And rightly so. I know I was when I first started going to the gym and I didn't go near it for years.
This machine is unique in that the more weight you add to the stack, the easier. It works by counter-balancing your own weight. If you weigh 130 lbs, and add 100 lbs to the stack, you'll be lifting 30 lbs of your own weight -- which is a good place to start. As you get stronger, add less weight to the stack and pull up more of your own -- until you're pulling yourself up completely without assistance.
Some machines are designed to be knelt on, others standing. Which ever you have, be sure to position your body to hang under the bar and mimic the natural movement of a pull-up - don't lean back.
A narrow, neutral grip (hands facing each other) will be easier and work the the mid-back. A wide grip will be harder (use more wight on the stack) and hits the lats hard.
It's a decent method to build up your pull-up strength, though I prefer using a large elastic band looped over something that you can put your knees or feet on.