Eureka! Thank you!
Earlier today, after I posted my help message, I spotted another message in the forum that suggested going at the audio file in a different fashion-- namely, on a Mac, hold down the Control key and select the audio file. A pop-up contextual menu appears with an option "Open With" at which point you can choose Express Scribe as the application and the file gets loaded. After discovering that, I played around with the various control features and everything seemed to work fine. My next step is rounding up a foot-pedal.
I was almost close to your solution-- I thought perhaps the files needed to be in a folder of their own so I did that-- but then went and tried again to select an individual file. Nada.
I just checked your solution-- works great!
Thanks again!!