There is no need to assign units when exporting an mfr file for Communicator since they'll be maintained automatically after the model is imported into Adviser. We can change the units that you're viewing the model and plots in after importing the model though.
To change the display units in the Ribbon, open the Application menu, select Options, and you may change the display units in Communicator between English and Metric right on the General tab. For the Classic interface, go to File, select Options, and change the units on the General tab. Unfortunately Communicator does not have the option to view the model in dual units like Adviser does.