Which Milliput do you have? The one in a white box with red trimmings? If so, that's the 'normal' one and old Milliput can be hard to knead and work. The darker putty snake often gets a 'crust' that needs to be removed, as this darker material will not mix at all, nor cure, leaving you with soft 'spots' into the cured Milliput...
Water is a good 'solvent' for Milliput, but unfortunatelly, it's only working when the mixed product is a few minutes old, and will not work with any of the two components before mixing...
Great stuff can be made of Milliput..! Don;t give up on it..!