For MPEG-2 playback and conversion, you need the QuickTime MPEG-2 Playback Component. You can buy it from Apple (
http://www.apple.com/quicktime/mpeg2). If you have either Final Cut Pro or DVD Studio Pro, then the component is already installed and you don't have to buy it; but you may need to download the latest version from Apple (
http://www.apple.com/quicktime/mpeg2/download). If you bought an older version of the component, you can update it to the latest version (
http://www.apple.com/quicktime/mpeg2/update). Note that the MPEG-2 component is neither required nor useful for MPEG-1 or MPEG-4 files. But it is required for VOB files and transport streams, because they are MPEG-2 files.