1. You have install telerik version 2012 in your system and also add this control into toolbox. Add this DLL into your project. By using this way you can able to see design view.
2. Now you have install the Telerik version 2013 in your system. Added this DLL version in your project. By using this way you can not able to see design view. DLL added in toolbox and in project.
Sometimes due to GAC it will create this issue.
Please follow this http://www.telerik.com/help/aspnet-ajax/introduction-design-time-troubleshooting.html
-> Uninstall both the version from your system
-> Remove the Telerik controls from toolbox
-> Restart your system
-> install the latest version
-> using configuration wizard add Latest DLL version in your project.
-> check Design view
Let me know if any concern.