I'm following your thread with interest as I'm trying to get madwifi working on my laptop.
As for finding it, have you tried the "locate" command? If not, as root, type "updatedb" and let it run for a while. This creates a database of all the files on your hard drive.
When that's done, you should just be able to type "locate madwifi" and it will give you a list of files and paths.
Please just ignore me if you knew this already and had tried it.