I had the same problem, finally re-installed iTunes and my iPod now appears in iTunes. However, when I tried to transfer some podcasts and books I got the message that the material could not be played on that iPod (6th generation).
Nothing happens... I went through all the possible options that I found on the Internet. When I plug my Ipod into my Mac it shows the Ipod icon on the desktop, and Itunes also opens, but Itunes does NOT show the Ipod there.