what you need to do is look at the threaded tree they have as an example. And notice the pattern they use for inorder (it is spelled out in the book, and they have some psuedo code for it). Anyways, once you get a good grasp upon that, then go ahead and look at how you traverse a tree in postorder. You will start to notice a pattern forming on how to traverse it using the threads. It isn't as clean a pattern as inorder is, but there is a patter nonetheless.

good luck -brett