I just followed the steps outlined by jav
at the top of page 2 of this thread. Creating the required url and idx files was just a case of running the perl script produced by jav against my asc input file.
My greatest problem has been producing a DVD that could by read ok in the car itself. I could copy my original DVD ok but as soon as I tried to edit it my edited copy would always display No Entries when I tried to display information on a location to see if my additions to the menus had worked.
In the end it was a case of trial and error to find a toolset that worked. For me that turned out to be DVD Decrypter to rip the original NAV DVD, UltraISO to copy in the edited and new files to the iso file and finally DVD Decrypter to write the modified iso file back to DVD.
If you want to outline how far you have got and what bits are working ok and which aren't, I'd be more than happy to try and assist you.