I am sorry for the trouble that you have encountered.
Visual Studio sometimes fails to load third-party components in the Toolbox due to various reasons. One of them is invalid Toolbox cache (stored in .tbd files). Resetting the Toolbox (right-click the Toolbox and click 'Reset Toolbox') or cleaning the Toolbox cache (.tbd files) usually helps with the issue.
I am glad that you managed to resolve the issue on your side. If you encounter any future problems don't hesitate to contact us again.
All the best,
the Telerik team