Double-click on the midi object to open the midi editor. Click on the Velocity button, top fourth from the left. This opens the velocity screen at the bottom of the midi editor. You should see some vertical lines. Dragging the line up increases the volume. The easiest is to click on the Velocity tool which is just to the right of the 3 pencils (Ctrl + 5), click on the note and drag up or down to change the volume and thus the height of the velocity line.
You can also draw the shape in the velocity editor at the bottom of the screen. I suggest that you look up Velocity in the Help screen for detailed information.
You say that Music Maker is not intuitive to use: like most complicated programs, you need to read the manual. Read the Quick Start chapter. The on-screen Help manual is fully searchable.
Context sensitive help: only tool tips show up if you have turned this on (on by default) under Help, Documentation, Display tool tip.