ScreenFlow User Guide
Media Capabilities — AVI

Video may not play

Some AVI files may not play the video (the audio will play however).

Workarounds

The work around is to convert the media into a different type (such as MPEG-4).

Try a codec helper such as Perian (http://www.perian.org), which helps QuickTime play a myriad of normally unsupported file formats.