Thank you for writing.
A possible reason for the observed behavior is a missing Telerik.WinControls.Design.dll in your Global Assiembly Cache. If you used our installer, this assembly should be automatically placed in GAC. Could you please make sure that it is installed there and if it is not, please provide me with details regarding the installation procedure of our suite that you have performed on your machine.
I am looking forward to your reply.
the Telerik team
Q1’11 SP1 of RadControls for WinForms is available for download
; also available is the Q2'11 Roadmap
for Telerik Windows Forms controls.