Ok, so then what you want to do is add a new item to the collection the RadTreeView is bound to. The RadTreeView will detect the change in the collection and then display the new item. Also, you are probably using hierarchical data, you'll want to make sure you add the folder at the proper level.
For instance, here's an example of adding an item to an observable collection. This isn't a RadControls question but a Silverlight one.
will bring you to the documentation on the Add method for the ObservableCollection class
will bring you to an example of someone who added a new item to a collection.