This article explains the most common issues and their solutions related to the Visual Studio Toolbox and the Telerik UI for ASP.NET AJAX controls:
Telerik UI for ASP.NET AJAX controls do not appear in the toolbox
Reasons and Suggested solutions:
Reason 1: The Visual Studio Extensions feature if the Telerik UI for ASP.NET AJAX installer was not installed.
Solution 1: Uninstall Telerik UI for ASP.NET AJAX and run the installer again. Click the "Custom" button in the Setup Type screen and ensure the "Visual Studio Extensions for Visual Studio xxxx" feature is selected for install.
Reason 2: Custom/third-party/Telerik exe-installer used to register Telerik controls to the toolbox, that prevent the ToolboxControlInstaller package of the IDE run correctly.
Solution 2: Right-click the ToolBox and choose Reset. You would need to manually re-populate the toolbox with the items you've added previously.
Regardless of the cause, you can always try:
Uninstalled the old Telerik UI for ASp.NET AJAX version, installed the new one, but the ToolBox contains the old item registrations
The Visual Studio ToolBoxControlInstaller package did not refresh the ToolBox items during the first start of the IDE after the installation.
Right-click the ToolBox and choose Reset. The ToolBox should populate correctly.
The Telerik controls toolbox items are missing or disabled.
Unplug your wireless mouse. More information is available here.