Hi Jorge Pascual,
There is a general problem with some complex controls in Visual Studio design-time. These problematic controls have been disabled so that they can not be modified by inheritors. This issue appears for Microsoft controls and Telerik controls. You can have a look at this blog
. It refers to Visual Studio 2005 but the issue persists in 2008 also. You will have to make your modifications through code.
the Telerik team