In the Dimension styles settings box the units are grayed out and they cannot be changed.
This is because AutoCAD does not have labels so this is a restriction of the DWG workmode. To have it work correctly use the following steps:Under the units category in the dimension styles dialog1. Toggle on working units2. Set the units to the desired MU/SU3. Set the labesl to desired MU/SU4. Toggle off working units5. Save the dimension style.This will then allow you to place dimensions with the master unit and sub-unit labels