When I signed up, I received my account approval, domain registration and FTP user emails within minutes of each other.
Since you say you 'added' a plan, I assume you did this through the panel using an existing account.
Are you using a valid FTP username/password when attempting the FTP log-in? This is different to the log-in credentials used for panel log-ins.
You should use the username/password of the machine user that controls the domain you wish to FTP into.
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