Without more information ie which version of Movie Edit Pro (MEP) and the video format of the video being added to the timeline, the most common cause of this message is that you are using and older version of MEP and the video you are adding to the timeline has Dolby Surround Sound (DSS) encodeed audio.
If this is so, the capability of importing DSS has been removed from older versions of MEP due to issues with the licensing conditions of the Dolby codec.
Use MediaInfo to analyse one of the video files that triggers the above download message, see this tutorial on how to set up and get the information required from MediaInfo for posting in the forum.