Click on the SET STEPS PER UNIT button, and a pop up window will appear asking which axis you would like to calibrate.
Select an axis
Another window will then pop up asking how far you would like each axis to move.
Enter a number (inches)
Be advised that when you enter a number it will then move the spindle that distance (or what it thinks is that distance.)
As soon as the movement is done, another window will appear to ask you how far the machine actually moved.
Enter in the actual distance it moved.
Mach will then suggest a number for your steps per unit. Accept it and Mach will automatically calibrate the axis to that setting.
You can also change the setting manually by going to Config/Motor Tuning