Operator->Edit Screen
Create or select the button you want
In the properties tab for that button's left up script:
local rc = mc.mcAxisSetPos(mc.mcGetInstance(),mc.X_AXIS,0.0);
rc = mc.mcAxisSetPos(mc.mcGetInstance(),mc.Y_AXIS,0.0);
rc = mc.mcAxisSetPos(mc.mcGetInstance(),mc.Z_AXIS,0.0);
HTH
RT