Adding new notes

For light MIDI editing, you can use the Internal Edit Mode. This mode lets you select notes, copy, move or delete them and alter their properties (see below). To add notes using the mouse, Left-drag. For more extensive MIDI editing, you may prefer Draw Mode. You can click or drag to add a new note, without having to hold down Ctrl. However, Draw Mode doesn't offer region-level editing.

So, to summarize:

Selecting, moving, copying, trimming, deleting regions	`Note Level Editing` disabled, using object, range or other mouse modes
Selecting, moving, copying trimming, deleting notes	`Note Level Editing` enabled, and using mouse object mode
Adding new notes	enabling "Note Level Editing" and then either using mouse object mode and `Left`-drag, or using mouse draw mode.

Note that there is also a step entry editor allowing to enter notes from a virtual keyboard and lots more besides.