Installing Mu on macOS is super easy.
Mu will run on any machine running macOS version 10.11 El Capitan or later. There is advice on Apple’s website on how to check your macOS version.
When you open the installer you’ll be asked to accept the software license agreement and then your computer will verify the package is not corrupted (this may take a couple of minutes).
Mu will install into the Applications folder on your Mac.
Double-click on the Mu icon. You’ll see a dialog to check you want to open Mu. Click “Open”:
This can take some time as OSX checks the Mu package, but you’ll only ever have to do it once.