The code posted in this thread are not Calibre-compatible recipes. They are PHP scripts for creating/downloading printable HTML or ebook versions of fanfics.
You can find a Calibre recipe for FanFiction.Net
here. Read the notes for recipe-specific instructions/tips. Refer to the Calibre online manual for instructions on how to add custom recipes. One caveat, though, is the recipe downloads a whole bunch of fanfics at the same time and saves them as one "omnibus"/collection file. Unfortunately, the current version of Calibre makes it difficult to get it to download files one at a time. Kovid has plans of creating a mechanism to download individual files in the future, but how soon he'll be able to implement it is unknown.
For getting individual HTML files which you can then convert to ebook format using Calibre, you can use
Code:
http://utterlyinsane.org/fflag/print.php?storyid=1234567
in case erayd's website is ever down. Obviously you need to substitute the actual story id for 1234567.
I know I promised a guide and clean/updated scripts and I'm really sorry I still haven't gotten to it. I just got hooked on watching Heroes and I'm currently distracted by reading Sylar/Elle fanfiction (yes, I'm twisted). I'm running out of fics to read so maybe I'll finally be able to get to working on the scripts this weekend.
As for Mac... I tried it (Hackintosh, OSX 10.5.5) and I found Linux (Kubuntu) easier to use. Sorry, won't be able to help in that area, aside from the Calibre recipe which should be universal.