As installed, it is a .pyo file (compiled, i think)
You will need the actual source files -
download the .tar.gz SOURCE file from the Calibre code repository
http://code.google.com/p/calibre-ebook/
The two files I pulled out of the .tar.gz were _init_.py & driver.py from the following folder:
\src\calibre\devices\nook
Copy them to where you have a similar folder on the mac (again, sorry I don't know where this is)
You do not need to delete the .pyo files in that folder on your machine. The .py files will over-ride Calibre when it starts up.
Don't have a clue where to look on a mac, sorry.