µ-µ diagrams do not currently work in THERMOCALC 3.50 (Dec 2020). It remains on the list of things to fix.
In µ-µ diagrams, chemical potentials replace components of the bulk composition as the independent variables. They are used where it is believed that gradients in chemical potential have been “stranded” during system closure. This interpretation might be posited for symplectites, for example.
Before trying to calculate a µ-µ diagram you should be confident with calculating pseudosections and phase diagrams in general. Once you’re ready for a µ-µ calculation, the following tutorial by Richard White should help. We no longer provide the input files, because they are now very outdated. However, if you want to reconstruct the later tutorial calculations, you can download modern-format thermodynamic datafiles for metapelites in KFMASH. In your scriptfile, do not set a bulk composition, and add scripts as directed in the tutorial.