First, be aware that if your machine is set up in inches, that it's 55" wide, and 40" high. Use the scale DRO's to change the size.
The 0,0 location is near the center of the skull. Jog to the center of you're board, and zero the X and Y axis.
Then jog the Z down until it touches the board, and zero the Z axis. Jog up a little, and hit Cycle Start.