how to add json library

i am new to python, on my Mac, when i issue command

User:ihasfriendz user$ python main.py
Traceback (most recent call last):
  File "main.py", line 2, in <module>
    import json
ImportError: No module named json

I get error on json. how to add this library? i'm using 2.5 (the default came with leopard)