You cannot just use the Media Kind tab to change a Music file to Podcast and have it appear in the Podcast section - it doesn't work that way.
If you do try this, as I and many others have, and the files seemingly disappear...
Connect your iPod to your Computer; now go to iTunes, now go to 'File > New Smart Playlist'
Now use some information you know about these files, i.e. Album, Artist...to locate the missing files.
For me it was my Ricky Gervais podcasts which were labelled as Audiobooks on my iTunes that I wanted in the Podcast section; I altered the Media Kind to Podcasts and they vanished, so I created a Smart Playlist on iTunes which searched my iPod for any files (even those that seemed lost) containing the words 'Ricky Gervais' and voila, they all appeared - I promptly highlighted all the files and used Media Kind (CTRL + I, Options, Media Kind) and changed them all back to Audiobook and they all now appear on my iTunes again.