There could be some software blocking the Kobo from accessing the computer if it doesn't reconize the device is even connected, does it charge when you plug it in or does it completely not even reconize the device at all?
Like sabredog suggested I'd try a calibre sync and see if the computer allows that or maybe try a different computer to see if it's that computer's issue. Was the desktop application off the kobo or the website
www.kobobooks.com/desktop