Clone this repo directly to your plugins folder. You don't have to take the plugins out of the MainMenu folder
This plugin comes with an example map, L_MainMenu, inside the main menu folder. When you play it, it should load the main menu from the BP_DefaultMainMenuConfigurator.
To make your own main menu, subclass BP_DefaultMainMenuConfigurator or the MainMenuConfigurator class and then set your Main Menu Configurator class in the Project Settings -> Game -> Main Menu Settings.
Once you've done that, you can use the main menu widget anywhere in your project.