Could you specify what the exact issue is? A quick look at the code makes me wonder if it even compiles.
Is it possible to assign a String to a Node?
I don't see an addright() method in the Node class you posted
Where is node N declared?
I don't see an addleft() method in the Node class you posted
If there is an addleft method does it return something that you can assign the String "d" to?