Option 1
Step 1: In a FTP client enter this information.
Host = add on domain name
User = addondomain@primarydomain.com
Password = what you entered during Addon domain creation
Port = 21
Step 2: Once you have successfully connected to your ftp client will be in the root of the add on domain name.
If everything was successful you should see a folder called cgi-bin.
Step 3: Upload your files.
Step 4: Enjoy your new site.
Option 2
Step 1: In a FTP client enter this information.
Host = primary domain name
User = cPanel username
Password = cPanel password
Port = 21
Step 2: Once you have successfully connected to your ftp client will be in the home directory.
Step 3: Double click the public_html folder. Double click the addon domain folder.
Step 4: Upload your files.
Step 5: Enjoy your new site.