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.
See About this document... for information on suggesting changes.