>>> from mongokit import *
Traceback (most recent call last):
    from magic import Magic
  File "build/bdist.macosx-10.5-i386/egg/magic.py", line 112, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/ctypes/__init__.py", line 353, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: dlopen(/opt/local/lib/libmagic.dylib, 6): image not found

これはfileが入ってないせいらしいので、portで入れとく

$ sudo port install file
--->  Computing dependencies for file
--->  Fetching file
--->  Attempting to fetch file-5.04.tar.gz from http://distfiles.macports.org/file
--->  Verifying checksum(s) for file
--->  Extracting file
--->  Applying patches to file
--->  Configuring file
--->  Building file
--->  Staging file into destroot
--->  Installing file @5.04_0
--->  Activating file @5.04_0
--->  Cleaning file

これでOK