Hello Divyesh Chapaneri,
Thank you for writing.
To address the first issue you need to add a reference to the Telerik.WinControls.UI.Design.dll.
To address the second issue you need to open the Property Builder from the smart tag of the RadTreeView and on the upper left portion of it, you will see a strip of buttons. The second button from left to right is the one you are looking for. Note that first you have to select a node in order to add children to it.
Please write back if you need further assistance.
All the best,
the Telerik team
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