The Piano Roll editor lets you view and edit your MIDI sequence. Horizontally, you can scroll through the MIDI events in time and vertically, you can scroll through the different keys/octaves. You can zoom in and out by pinching with two fingers horizontally or vertically.
Just as the timeline/main screen, the Piano Roll editor screen has different 'modes' which you can select using the buttons on top. For creating new MIDI events, select the Draw mode as displayed in the screen shot above. The left panel will change and presents the following options:
The Event type allows you to choose which kind of MIDI event you want to create. Besides notes, you can also enter Note Velocity (the volume of individual notes), Control Changes, Program Changes, Channel Pressure and Pitch Bends.
(to be continued)