Have you saved your Customer project in the Areas folder of the DemoPlug project, as the tutorial indicated? If yes, the you should modify your action method to include the area name.
@Url.Action("Index", "Customer", new { area = "Customer"})
To do the reverse and call an action in DemoPlug from the Customer Area, try this:
@Url.Action("MyAction", "DemoPlugController", new { area = "" })