cannot open "*.vob" in Ulead's DVD Workshop

[Hundewetter]

I downloaded some thx trailer (vob files with doby digital 5.1 sound) and want to create a dvd. To do so I also downloaded trial versions of ulead's dvd workshop and video studio 6. The problem is that I cannot open the vob files with these programs. I downloaded some vob filters so that even windows mediaplayer could play the vob files (but quite slowly and with some graphic errors, sound seems to be okay). Then I tried to deal with avisynth but without success.
Does anyone know how to fix this problem? Are some codecs missing? Are there better programs for dvd authoring (that can deal with vob files)? If you have any idea please let me know.

Stefan