How to change Podcast default Directoy?

You can change the directory by opening the itunes preference pane in library/Preferences/com.apple.iTunes.plist in your home directory. I'm not sure exactly which directory needs to be changed, but I changed all of them. If that still doesn't work, then you can put the iTunes directory where you want it (normally Music/iTunes in your home directory) whereever you want it, and use the Terminal command "ln -s path-to-where-you-moved-iTunes ~/music/iTunes" If your not sure how to type in the path, type in / and then press the tab key until you see a listing that contains the folders in Macintosh HD (mine shows the following):
.Spotlight-V100 Network dev opt
.Trashes System efi private
.vol Volumes etc sbin
Applications automount home tmp
Developer bin media usr
Library cores music var

From there, navigate to the directory you want, using '/' as a directory seperater. Other drives are in the /Volumes folder. Some characters like space need to be prefixed by a '\'. The command I typed was "ln -s /Volumes/HOME/music/iTunes/ ~/music/iTunes"
I tried to give that little tutorial for people who don't know how to do terminal, not sure how well I did.

You canot change just the podcast directory.
Everything is stored in the iTunes music folder.