Hi Ole Kristian,
Thank you for contacting us.
You need to follow these steps to remove the error:
1. Open the project
2. Add the Telerik.WinControls.UI.Design.dll
in the reference
3. Save changes and close the project
4. Open the application and rebuild it.
I also attached a video (drag and drop it over your browser to play it) which demonstrates the approach.
The error is caused when have installed one version of controls in GAC and the project references another version of the dlls. If you use RadForm or if you want to make changes at design time, you need to add the Telerik.WinControls.UI.Design.dll when working with version different from installed one.
I hope this information helps. Should you have further questions, I would be glad to help.