If you're getting the error:
PHP Fatal error: Class 'SoapClient' not found
.. it means the PHP SOAP client isn't installed on your server.
The Smart Send API is SOAP based, and the SDK and Smart Send-produced plugins use the default PHP SOAP client; however, it's not installed by default on some hosting environments.
Ask your hosting service to install it, or see http://php.net/manual/en/soap.installation.php