On some Sony readers you need to install the Sony Reader software even if you don't intend to use it.
That is because the installation also installs a driver that is needed for ADE to see the Sony.
The Sony will show up as an external drive without the special driver, but not as an ADE device.
(Not sure why this is needed, most eReaders don't need a driver for ADE, but some Sony's do).
Try using the older ADE v1.7.2; especially if you are using library books.
There are bugs in ADE2.0 that prevent some Sonys (eg PRS-350) working with ADE2.0 and Overdrive.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172