Hey, Sometimes when I convert to avi files, it either won't play cause of the size not being right or it's not the right codec. The codec should be called xvid. If you already are using a converter, make sure that the codec of the avi is set to xvid and that the size is correct. xvid is just the type of avi video file. Either way, there's a link at the bottom for your possible solution. I have a mac too. Mac's are superior in every way to pc's. The only issue is that only the best products are compatible so there's only so much variety.
If you know of an mtv video converter for mac, let me know.
Mac AVI Converters - AVI Converter Mac OS X software, convert AVI on Mac