You've got the Sony Reader Library software loaded on your Mac, right?
The current version actually handles the Overdrive DRM for you.
When you download the ebook from Overdrive to your Mac use the "Reader Library" location in addition to the location for "Adobe Digital". Overdrive will let you download a book in your digital cart more than once.
Hook your Sony reader up to your Mac, start the Sony Library software and set up the Sync functionality between the Sony software and the reader, then let the Sony software put the ebook on your reader.
If you finish the book early you can return it using the little green arrow next to the title in ADE. I haven't found a way to do it using the Sony software yet, though, which is why I'm downloading to both ADE and the Sony Library.
Clear as mud, right?
