You need to make the Silverlight plug-in windowless. There are two ways to do it, depending on what is your host page (ASPX or HTML):
1) In your ASPX, you should have:
<asp:Silverlight Windowless="true" runat="server" ...
2) If you are using an HTML page, you should add this parameter to the Silverlight OBJECT element:
<param name="windowless" value="true" />
the Telerik team