Hi Keith.
Thanks for this. I ran into the same problem, and the info in your post led me to the solution. Just wanted to clarify one thing, though-
The first button to hit is Build/Format (not Build & Burn as you stated*), and that pops up a small window. In that window, click the Disc/Volume tab, where you’ll find that the single layer dvd is the default setting, and you just need to change that over to dual layer. All the other settings in that window can remain in their default position (unless you really want to customize it, and you know what you’re doing). Then, make sure you have a blank dual layer dvd in your burner drive, hit the Build & Burn button at the bottom of that window, and you’re off and running.
*I’m running DVDSP 4.2.2, dunno if it’s called something else if you’re using a different version than me.