So, I've recently purchased a few clips, i think all of them from Jayce & Jilly's store, that I cannot seem to convert. I downloaded Flip Player, suggested for MacBooks, and now can see mp files, but the avi's won't convert.
First off you can play most any video file with VLC player which you can get for free here: http://www.videolan.org/vlc/index.html. But if you really need to convert an AVI then Handbrake claims to handle most formats, also free: https://handbrake.fr/. From what I can tell though it only converts to MP4 or MKV formats, and there's no way to convert just part of a file; it is good for compression though (lowering the bitrate). The conversion program I use the most is WinFF http://winff.org/html_new/downloads.html (also free) which can ouput many formats and can be used to process part of a video. However, WinFF only works on Windows and I'm not sure that's useful to you HTH!