Hi,
yes the gnomon is always at 0,0,0 but that means your part toolpath starts at x=250, y=250 and z=250 ie outside your boundaries.
You need to (in Mastercam) highlight the whole of the tooplath, stock and so on and translate it relative so the first corner of the part
is not 250,250,250 but 0,0,0.
This is a CAM issue, not Mach3 at all. You have drawn your part too far away from the WCS origin. Shift your drawing to be close if not
coincident with the WCS origin.
Craig