Have you tried to add the whole DVD at once? (by dragging the drive icon) This works for example with normal audio cds.
i have the same problem v1.1.9beta1(tested with columns ui or default ui)-w7x64sp1
In my tests, drag-n-drop to a playlist from a CD-R via Explorer was possible with Columns UI, but not the Default UI. Windows XP, foobar2000 v1.1.13. I searched but could not find an explanation.