To upload files using an FTP client, simply connect to ftp.your-domain.com.  The username will be admin@your-domain.com and the password will be the password which was supplied to you.  The login screen will differ based on the FTP program you're using, but it should look similar to the graphic below.  Please note that SFTP and FTP over SSH2 are also supported.  If you have an FTP program that supports these alternate protocols then please use them.