How do I enable mail notifications for an event?

You can enable mail notifications for an event by registering it with the \Aero\Events\ManagedHandler listener:

class ServiceProvider extends \Aero\Common\Providers\ModuleServiceProvider
{
    protected $listen = [
        Acme::class => [
            \Aero\Events\ManagedHandler::class,
        ],
    ];
}

If you have implemented the register method of the service provider, ensure it calls parent::register() else the listeners will not fire.

Now your event class will be selectable when creating a Mail Notification in the Admin.