Jack-
When you create an addon domain in cPanel, there are 3 boxes. One for the domain, one for the username, and one for the password.
This is a unique FTP username and password for this specific site. This is what you should configure in your Dreamweaver site manager to keep the sites separate and organized.
You shouldn't need to use any redirects in the addon domain manager, just fill in the three boxes and you should be able to FTP to the proper folder inside of the main site's public_html
The addon domain username will be in the format of user@domain.com and not just cpanel_user like the main site is. This is where most people get hung up.
Not sure if you have seen this thread, but it may help:
Addon domain FTP username