I have resolved the issue. I was missing the Packages thing while creating the instance. As I had a static method in my class I have to call it in the following way. (parseJson is my package name and ParseJSON is my class name) . Here Packages is the default class provided by Rhino
var call = Packages.parseJson.ParseJSON;
xyz=call.processJson(data);
If it is a non static method I had to create an object in the following way
var call = new Pakages.parseJson.ParseJSON();
xyz=call.processJson(data);