G54 is the default work offset and is the value you normally see in the DROs in Mach. The only time you wont see G54 there is if you have called one of the other 252 offsets or you have pressed the machine coordinates button to view them.
Short explanation of machine versus g54 would be
You start Mach and press Ref All and your machine homes and sets the machine coords, usually zero. You then jog to where you want your work offset zero to be and zero the DROs, you have now just set the G54 offset. There are another 252 offsets that can be called and zeroed in a similar fashion but mostly you will be in G54
Hood