I took a quick look at your account - looks like the site in question is set up with Custom Hosting, so you'll want to connect via SFTP directly to the domain name -- username should be 'admin' and the password should be the same as for your main Sonic account. The main website directory should be 'public_html'.
For Personal, Value, and Basic Hosting, you would connect via FTP to ftp.sonic.net using your Sonic username and password. Website files are under the 'public_html' directory, with each site either having its own folder (the name should be the same as the site's domain name) or for a users.sonic.net site, everything will be right in 'public_html'.
-- Joe M
Sonic System Operations