Hey,
I think I can help you with this. I'm the Electronic Resources Librarian at SFPL. I deal with a lot of OverDrive questions. Let me see if I can answer the questions from your latest post and then if you still need assistance, I'll leave you my contact info.
The two ways to download audiobooks is 1) Library to iPad, with the OverDrive Media Console app and a WiFi connection and 2) Library to Mac/PC using the desktop version of the OverDrive Media Console software and an Internet connection. I asked OverDrive about the download error you are getting and they think it might be related to the security settings on the WiFi connection you are using. In particular, they have found downloads from the OverDrive servers are not permitted if you are using a public WiFi connection.
You should be able to download an MP3 audiobook from the Library to your Mac as mentioned in #2 above, then open iTunes and add it to your iTunes Library.
Once you've downloaded the title using the OverDrive software, go to iTunes and select "Add File to Library..." from the File menu. Unless you've designated another folder for your OverDrive files, the default on a Mac should be: Documents\My Media\MP3 Audiobooks. The file will be added to your music collection so look for it there.
Here's a link to the help file on the MP3 format -
http://sfpl.lib.overdrive.com/Help-FAQ-Format425.htm
Anyways, I hope this helps. Like I said, if you still can't get this to work for you, contact me at jgarcia@sfpl.org or 415-557-4478.
Cheers,
trent