We have previously explained how to find the preferences file for the Dreamweaver site definitions using a Mac. With Mac OS X and the new Dreamweaver MX, the site definitions have moved. Here is where to find them now.
/Users/<username>/Library/Preferences/Dreamweaver MX Prefs
This is the file that contains your FTP info and your passwords. Making a backup copy if you have to reinstall DW can save you a lot of time.