This is a known issue with Sitefinity 3.0 and we are still investigating it. You cannot edit Sitefinity .master pages in the design view, because they are using our CmsForm implementation, instead of the standard ASP.NET HtmlForm. But there is a workaround available for this problem - you can replace the CmsForm with the ASP.NET HtmlForm in the master page, edit the master page in the VS.NET and finally set back the CmsForm.
Replace:<sf1:CmsForm ID="form1" runat="server">
with:<form ID="form1" runat="server">
Hope this helps.
the Telerik team