You can install this plugin into your CakePHP application using composer.
The recommended way to install composer packages is:
composer require CakeMojo/Messages
If you want to use the provided Messages table:
bin/cake migrations migrate -p CakeMojo/Messages
Ensure the plugin is loaded in your config/bootstrap.php file
Plugin::load('CakeMojo/Messages', ['routes' => true, 'bootstrap' => false]);