To place a note in a Melody Window, click on the Droplet Tool in the top left corner of the main window, then click anywhere in a Melody Window. You will see a note appear in the window.

The note may be resized by dragging the handles at either end.

Melody Windows are monophonic? that is, they only support a single melody line.  If a note is placed on top of or below another note, the notes it overlaps will appear greyed out UNLESS Polyphonic Melody is enabled.

On deselecting the new note, the underlying notes will be overwritten. If Polyphonic Melody is selected, the notes will be added to the original notes.