The assisted pull-up machine targets many muscles, the same muscles as normal pull-up. This includes the Latissimus Dorsi (lats), biceps, trapezius (traps) muscle, deltoids, pectoralis major (pecs) and the teres major, minor and the infraspinatus.
Remember this is likely one of the only machines you will find in a gym where going heavier makes it easier, and going lighter makes it more difficult. Dependant on where you grip the machine will also have an effect on how it feels, generally the wider you go the harder it will be, the closer a bit easier.
To start, place your hands on the top bars and step yourself up nice and controlled. Choose an appropriate weight and put your weight onto the platform, lowering it (and you) down slowly. As you reach the bottom focus on bringing your shoulder in line with you ear. From this posistion, pull yourself up driving your elbows down towards your pockets and your chin in line with your hands. Ensure the movement is nice and controlled, holding for about 3 seconds. To come off the machine, place one foot on the side rest, and elevate your knees to allow the platform to raise back to its starting position - allowing you to step off.