how I resolve Zillow API Account not authorized Error?

Question

SimpleXMLElement Object ( [request] => SimpleXMLElement Object ( [address] => SimpleXMLElement Object ( )

        [citystatezip] => SimpleXMLElement Object
            (
            )

    )

[message] => SimpleXMLElement Object
    (
        [text] => Error: this account is not authorized to execute this API call
        [code] => 6
    )

)