Hi,

Where is this file ? (defparameter *default-notation-editor* "MuseScore 4.app")

Therew is not a specific file for that.

Pleace the expression in the 'User Functions.lisp' file or any other file which is part of the Extensions directory.

like this?
what should i do next?

yes

Merci Stéphane:-)!

Nothing more to do.

From now on the default notation editor on your system is Dorico 5.