There are several navigation-tree smart clips on the Flash Exchange site -- look for Flash Exchange under "Downloads" on the right hand side

I made this navigation tree with one of the smart clips. The smart clips include complete directions.

You also need to make an XML file that defines the tree, and keep it in the same folder as the Flash movie -- just type it in notepad and save it as whatever.xml
view the XML file
You might want to print this file and study the logic -- you can add or subtract as much as you want from the tree -- just keep the logic -- you never have to touch the movie -- it is easy.