The messages are standard fare; neither the .LAY or .LAYOUT files are ever included inthe final build. I'd suggest rebooting your Mac then starting a new DVD Studio Pro project. If that results in the same problem, you may need to trash DVD SP's preferences with this app: http://www.digitalrebellion.com/prefman/
Does the simulate funciton work? You say it plays fine in the viewer, but you may just be playing the track rather than simulating the dvd behavior.
David's steps are a good idea. Once you've done those rather than burning the disk within dvdsp, in DVDSP try doing a file: advanced burn: build. Create a new folder with the name of the dvd and DVDSP will create the video_ts and audio-ts folders in that folder.
You can than test the build using the mac dvdplayer by launching the dvd player and going file: open dvd media and navigating to the folder you created above.
This will save you alot of time and money on blank dvd-roms.