For both the MUX effect plug-in version as the MUX synth plug-in version you can setup a MUX preset file that must be loaded whenever you plugin MUX Modular:
For the effect version: Save a "Startup Effect.Mux" in your user/library/mux sub-folder.
For the synth version: Save a "Startup Synth.Mux" in that same sub-folder.
Note: Don't save it as a FXP (=VST file), save it as a MUX file using the Save As button in the MUX editor itself.
Then navigate to your MUX folder /User/Library/Mux then click New File and enter "Startup Effect.Mux" or "Startup Synth.Mux".