Please check if the Telerik.Web.UI.dll file is in the bin folder of your project. If it is there, you might need to remove the Read-only attribute from the physical file before you try to compile your project. Visual Studio might need to overwrite the file when you drag & drop the control from the toolbox and if it cannot, it will complain that the file cannot be found - even if it is in the bin folder.
Moreover you can try to add your assembly manually when go to the new Prometheus tab in the toolbox, right click and "Choose Items...". Then on the .NET Framework Components tab click "Browse" and select the Telerik.Web.UI.dll file.
the Telerik team