Many times a video file that you want to use to create a DVD is in a different format than the other video files you are using for your project. For the vast majority of DVD projects, it is best to convert all of your video files to the mpeg2 format.
There are many free programs that will convert your video files to mpeg2. One of the best is a small program called Handbrake. Use it to convert each of your video files to mpeg2 first, and then import these newly converted files into your DVD creation software. It will make for a much easier time making your DVD.