I prepared a sample video
for your showing how I create a RadTabControl in Blend and successfully view its RadTabItem`s Contents. You can also let us know the exact steps in reproducing your issue and we'll try to advice you better.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!