Thanks for using the plugin! Before you can get to creating and using your custom HUDs, you must install the plugin and it's dependencies. You can do this by dragging the .jars into your plugins directory. You can now startup your server and let HappyHud generate the default files.
Now that your server has started you should see a folder named HappyHud in your plugins directory. This is where all the configuration for the plugin will be stored. This folder should look something like this.
If you were to join the server at this point the display would not work because you do not have the resource pack. You need to create a resource pack and then drag the contents of the HappyHud/built-pack folder into the assets folder of the new resource pack. Most of the time whenever you make a change to the assets of HappyHud, you will need to re-merge the newly generated pack. Once you have completed these steps your pack should be working.