[GeoffR]

Calibre 1.5 should know about firmware 2.8.1 already, so something else must be going wrong in your case, especially since you are not able to drag and drop a book onto the device either.

Note that you don't need to convert the book to kepub format before transfering it, and it shouldn't be necessary to add any plugins to get Calibre to work with Kobo devices.

There have been some reported problems with Windows XP systems communicating with some Kobo devices. Also a damaged cable or faulty USB port can cause problems. Both of these issues would affect Calibre as well as direct drag-and-drop transfers.

Edit: You can load books onto the Aura from Calibre via wifi, without plugging in to the USB port. It might be an idea to try this, if the device still doesn't see the book then the problem is either with the device or the book, not with the USB communications. To do this, start the content server on Calibre, note the address/port it is using, then use the Aura's web browser to vist that address: port and download a book. Once you exit the browser the book should show in your library.