On the picture of the electronics,
http://wiki.ultimaker.com/File:El1.5.4-PCB.jpg
, look at the top/right.
See the 3 little circuit boards plugged into the big board? Those are the Pololu stepper drivers for your X, Y and Z motors.
On the picture, below each of the stepper drivers, are 3 little black rectangles. These are the jumpers that control how many "steps" it takes to turn the motor one revolution. Notice that the jumpers for the rightmost stepper driver (Z) is set differently from the other two - if you look really closely, you can see that the jumper (the little black plastic thing) labeled MS3 is only attached to one pin and that the other jumpers are attached to two pins.
CAREFULLY, with all the power off and unplugged, check the jumpers on your board to make sure they match the picture. To remove a jumper, just wiggle it gently with needle-nose pliers of something similar.
Don't hesitate to ask if you have more questions. Pictures are good, too.
Thank you very much! very helpful! fixed!