With 9.1, unless someone has a gun to your head, you should be using Feather MVC widgets :) Webforms ascx are bloaty and overly complex compared to MVC. Also an MVC widget will register itself in the toolbox.
Regarding this though, did you BUILD your project after you added the usercontrol? If you did build, can you paste in the corrospoding Hello line from your Toolboxes.config?
Either way though, thunder should be able to generate the stubs and register it for you.