Every Time i tried to connect my sitefinity Project to thunder it gives me error "Unable to connect"
To add a custom MVC widget you would scaffold an MVC widget as normal. To get it to show up in the Toolbox you need to decorate the controller with this:
[ControllerToolboxItem(Name = "Features", Title = "Features", SectionName = "MVC")]
Here is a help article. https://docs.sitefinity.com/for-developers-create-mvc-based-sitefinity-widgets
If the default Visual Studio MVC scaffolding is not working on your project try this article, it's a little outdated but might still apply: