Hi hand-up-not-out,
While you cannot upload an MPG file into your Auctiva account, if you are able to generate html code for this video file, you will be able to place that in your Auctiva created listing.
In order to accomplish this, you just need to click the "Switch to text mode" link above the description editor and paste in the html code that generates your video file.
I'm not quite sure what it would take to create html code for a video file, so it may require some research on your part to figure that out. Or, perhaps someone who knows how will come along and post to this thread.
-Mike