The DockingManager component has been obsolete for the last two years and we removed its implementation in Q1 2011. I would suggest that you downgrade to Q3 2010 SP1 where the implementation of the DockingManager still exists and update the DockingManager to the new docking component - RadDock. You can use the Upgrade Wizard that we have prepared for switching from DockingManager to RadDock. I am attaching a screenshot for better understanding.
If you have questions during the process of updating to RadDock, feel free to write back.
the Telerik team
Q1’11 SP1 of RadControls for WinForms is available for download; also available is the Q2'11 Roadmap for Telerik Windows Forms controls.