Hi,
Just put your AVI files in a folder.
Files must be named nameofgame.avi , ex (pacman.Avi)
Then with AtomicManager on the folder tab make point the SNAP video folder to that folder.
Even if you have the .AVI file , you MUST have the .png file in the Snap (not the video one) folder.
then you must have the right code installed on your machine. It must be codec for windows mediaplayer 6 or < .
If you look in Atomic 0.14 there is folder called "video for loading screen" , inside there is an AVI file , If you use a tool to know what is the codec used by that file you could use that one to encode your video.
To make your video you can use CamStudio software (do a google search).
Now, i'm in the U.S (North Carolina) for still a week so i didn't have access to my computer to help you more. But i hope it helps you enough to start.
If it works, Atomic will display the video if present if not it will display the snap and if both are missing it will display the "missing snap" screen of your layout.