Check the following:
1. The software supports the video format.
2. The source file can be played on the Mac.
3. The video and audio stream are both included in the source file.
4. The duration of the source file has to be more than 7 seconds.

If you can play the file with VLC player or Quick Time player, you may need to re-code the file. To do that, please follow the steps below.

VLC Player: Open VLC, choose File>Streaming / Exporting Wizard from the top menu. Transcode / Save to file, follow the steps to re-code the file.

QuickTime Player: Open the file with QuickTime player, choose File>Export from the top menu to re-encode it to another video format.

After re-coding the video file, import the file to DVD Creator for Mac again.

If the instruction above doesn't help, please contact us with the video file for further assistance.