DVD is in mpeg2 format. For QT to play it you will need to purchase the mpeg2 codec. Alternatively, if you're just dwnlding to view on your computer, you can get either of 2 free video apps which will play it.
VLC (click for link) which will play various formats incl divx, mpeg1/2/4, dvd & vcd. Another one is
MPlayer . With either of these players u won't need to purchase that codec & you'll be able to play those dvd's. Just as a point about QT, go to your Sys Prefs & click QT. Then click on Plug-ins & down the bottom prss MIME Settings. In this new window click all of the codec plug-ins u want QT to handle. Hope this helps.
Happy video viewing.