Ok as a test zero your Z Axis DRO then go to MDI page and type G43H1 and press enter and see if the Z Dro changes, Next type G43H2 and enter and see if it changes, G43H3 and enter and see if it changes.
If it doesnt then sounds to me like your tooltable in Mach is not set up, if it is then I will get you to attach your xml, I will explain how to if the need arises.
Hood