Sorry for the caps, gents, but I'm sure there are plenty of Mac users out there looking for the solution to this conundrum.
Yes, it's quite annoying that we waited so long for the Sony eBook Library to come out for Mac and then when it came out it was incompatible (or seemed to be) with Adobe Digital Editions, one of the major formats out there. Well, in fact, or anyway it seems to me, Adobe is at fault!
It's not Sony's fault that Adobe Digital Editions fails to recognize your Sony Reader on the Mac.
It is to Sony's credit, however, that in your Mac Sony eBook Library Software you can now import the files from the Adobe Digital Editions. Here's how with a picture to show you the process.
In eBook Library go to File > Import Files. On my computer they were located under Documents > Digital Editions. Actually I only have one to date, but anyway, that's where Adobe stores them. When you import it, it will ask you a few things, for instance to authorize with your Adobe account and so forth. This stuff is all automated, and if anyone would like to post pictures of their first time going through this process (for the benefit of others), that'd be a good thing.
Well, there you have it. Once the book was transferred to my eBook Library I was able to put it on my PRS-505 with no problem.
Here's the beautiful result (Cormac McCarthy's
The Road purchased at Shortcovers via Adobe Digital Editions):
Happy about this. Not quite ready to let this old piece of hardware go.
Hope this helps people!