You have to set the Target property when you create the Nodes. For example if you bind the Tree View to a dataSource, you can handle the NodeDataBound server event and set the Target property of the node there.
It should be set for every node. Also, you shouldn't handle the NodeClick event if you want a navigation to happen in the iFrame of the content pane. With set Target and NavigateUrl it will automatically navigate in the iFrame.
If you still are not able to resolve this, please, open a support ticket and send us a simple runnable sample project, in which we could implement it.
the Telerik team