Then install MacPorts. Something like Go > Utilities > Terminal
xcode-select --install
xcodebuild -license
or https://www.macports.org/install.php I guess? After it installs go to it in Applications, open it up to accept the license agreement thing.Install or update x11 https://www.xquartz.org/
Then in the terminal
sudo port install mypaint
Then when it finishes Go > Applications > Macports folder > MyPaint !!!