When you create an add on domain in your cPanel you will be asked to select a password. This password, together with the username created will be your username and password to access the add on domain via FTP.

However, to simplify things and to avoid you having to remember/store many different sets of FTP details every time you create an add on domain, simply use your primary domain ftp username and password. Once connected, double click on your public_html folder to open it - locate the folder that is named for your add on domain, double click on this to open it and upload your add on domain website data to this folder