To upload to an addon domain with an FTP client you have 2 options :
1. login via FTP to your primary domain as normal using your main cpanel login details. Your addon domain files should be placed in the folder corresponding to the name of your addon domain.
2. login via FTP to the addon domain using the ftp username and password created when you setup the addon domain. As an example, if your primary domain is exampledomain4u.co.uk, your addon domain is anotherdomain.co.uk and the username you chose when you added the addon was "anotherdomain", the ftp login username would be firstname.lastname@example.org and you would use the password you chose when you created the addon.
We recommend that you use option 1 above, unless you are giving access to the addon domain to someone else. You can also create new FTP accounts using the FTP Manager in cPanel for this purpose. The username format for additional FTP accounts is always as above - username@primarydomain.