What format are your DVDs in if not mpeg2?
They probably are multiple .vob files which could have mpeg-2 video muxed with mpeg-2 audio or ac3. I think the OP wants these as a single muxed file.
I would give MPEGStreamclip or the other software a try again, but this time first copy all the files on the DVD to a folder on your computer and then work with them.
Is there lots of free space on this computer? Is the drive Mac formatted? You could end up with a file that's nearly 8 GB in size if the DVD is dual layer.