If I go to cpanel ftp manager, add a ftp account named "test" with access to /test How do I log in? test nor test@mydomain.com works :(

should be

host: ftp.thedomain.com
username: test
password: whatever pass you set when you created the account

It's not :P

the username is: account@domain.com

if its not working, then you are typing something in wrong