Tonescape's Musical Piece window is where the various different windows showing the musical notation of a piece appear.
Version 1.0 of Tonescape™ provides Pitch-Height™ notation in the Notation View. The next release will include traditional pythagorean/meantone staff-notation, and future versions will also include HEWM, Johnston, Monzo 12-edo-staff, Monzo quarter-tone-staff, and Sagittal notations.
The first thing to do is to make the environment suitable for composing your piece. You may hide the "Tuning Editor" window, since it's not needed here: right-click on the title-bar of that window, and select "Hide View". Also, you'll probably want to make the Musical Pieces window the largest one in the environment: resize it by dragging the sides of the window when the mouse cursor turns into a black double-headed arrow.
next: c) the Lattice window