Wow, 37 views and no answers!
Fortunately, I did some spelunking through the various forum topics, and found mention of the .mp4 format. Taking that as a big hint, I tried to use Visual Hub for the conversion, but discovered two things: One, there's an update available that I couldn't download; two, the developer no longer supports this product, and three, it would have only worked on a video that was less than five minutes in length in demo mode (the one I needed to convert was about 20 minutes).
I then used Handbrake to convert the .flv file to .mp4, and was able to load and play the video through Express Scribe.
As I didn't need the video to be full screen, I'll have to be a bit more selective with the conversion process; it went from a 1.5MB .flv file to an 80MB .mp4 file(!!) So, probably further processing with iSquint might have helped.
Hopefully this will help someone else who is curious about how Express Scribe handles video on a Mac!
--gdw