The navigational functions in PrE's DVD-Video authoring module are very limited.
Pretty much, one can choose a Menu Set (both a Main Menu and a Scene Selection Menu), and then with the addition of a Menu Marker, and Scene Markers, let PrE create the linear navigation. One can modify many Menu Sets, but only to a limited level.
For more intricate navigation, I strongly recommend another program, just for the authoring. Many users sing the praises of Sony's DVD Architect, which has a much better feature-set, than does PrE. I use Adobe Encore, but it is ONLY available with PrPro, and not as a stand-alone any more.
Steve Grisetti, our MOD here, has written a great book on DVD Architect, in the Muvipix.com series, and if you do go with that program, I highly recommend his book.