The Score parameter in the Region inspector controls whether MIDI notes in the selected regions appear in the score. If Hide is chosen in the Score pop-up menu, notes (and other content) in the selected regions are not displayed in the score at any display level above the region level.
In the Region inspector, choose Hide from the Score pop-up menu.
You may need to click the More disclosure triangle to see the Score pop-up menu.
Choose Show from the Score pop-up menu to have the regions appear in the score display.