Error in JSONHelper

In my code of WCF service , while using the JSONHelper , I am getting error of "JSONHelper  does not exist in the current context."
Is there any reference or any package which I am missing. Although Newtonsoft.Json is already added.
This is the code where I m getting error.
PNRStatus resObj = new PNRStatus(); resObj = JSONHelper.Deserialise<PNRStatus>(finalResponse); return resObj;