From MusiCAD

You can perform almost any action in MusiCAD with the mouse. During design, however, it has been assumed that most note entry operations can be (much) faster with the keyboard.

A number of things are, of course, sometimes much more convenient to solve with the mouse:

Of course you won't learn shortcuts by heart for those things that you don't need much, and the mouse is more convenient, but those things that you do use a lot can be done better/faster with a shortcut.
In particular, enter notes are really much faster with arrow keys and <Enter> than with the mouse.

In page view there are a few more useful mouse options.

Click on the:

If the mouse (mouse mode) button on the button bar is pressed, clicking a note will only cause the cursor to move.

To indicate that clicking with the mouse will enter a note, a red note will be attached to the mouse cursor:

mouse cursor having a red note

You can assign your own commands for the notation bar to mouse clicks left/right/middle - whether or not in combination with shift, control and/or alt keys. You use hotkey definitions.