If that's enabled you should be able to login via any SSH client. The button corresponds to the SSH client embedded in Plesk, which is not necessary to login via SSH, but might be handy for clients that don't have an SSH client themselves (although downloading PuTTY is easy and non-Windows OSes usuaslly come with an SSH client). This is a Plesk component you can install through the autoinstaller or Plesk Updater.