You can create a desktop shortcut by doing the following on your Mac:
1. Open Finder by clicking the icon on the bottom far left of your screen.
2. In the Search field, type "composer.exe" (or "musician.exe") without the quotation marks, and be sure to click the "This Mac" option (the default is "All my files" and it will not show up there because it is a program executable).
3. In the file list that comes up, click to select "Composer.exe" (or "Musician.exe"), then click "Action -> Make Alias" from the menu bar at the top of the Finder dialog.
4. Click the newly created "Composer.exe alias" (or "Musician.exe alias") file and drag-n-drop it to the desktop.
5. Optionally, rename the alias to "Notation Composer 3" (or "Notation Musician 3") on your desktop.
Now when you double-click the desktop shortcut, Notation Composer (or Musician) 3 will automatically start.