You should add references to
Telerik.Windows.Controls.Input.dll and Telerik.Windows.Controls.dll assemblies in order to use RadDateTimePicker. Please check
our GettingStarted article
for more details.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!