Perhaps AVI is problematic on a Mac?
As for actually seeing the file... there could be a number of reasons.
I would eliminate the following first:
Make sure filename is actually .AVI and not something else OT Browser won't recognize on it's look up list.
A workaround to this is often to change what the browser is looking for to *.* or 'all files'.
Somewhat less likely would be OT's setting to monitor and update file listings for the browser.
At one point I turned that off thinking I could save some computer processing but not having the file listing refresh was frustrating because files I knew were there were not being updated.
So, double check that setting.
Back to the original question, we definitely need to know more about the type of video you are working with. It's hard to troubleshoot without that information short of saying, "convert the video to a format OT will recognize and try again."