MacPython honours all standard Unix environment variables such as
PYTHONPATH, but setting these variables for programs started
from the Finder is non-standard
as the Finder does not read your .profile or .cshrc at startup.
You need to create a file ~/.MacOSX/environment.plist.
See Apple's Technical Document QA1067 for details.
Installing additional Python packages is most easily done through the
Package Manager, see the MacPython Help Book for details.