web api call return unauthorized (401) error

I am trying to call Urbanbuz web api for loyalty program. The api is returning : 'The remote server returned error Unauthorized(401) error' or 'Bad request(400)' or 'Bad request(500)'. This api uses key and secret string authentication

The code which am using is

var api = new API("http://ift.tt/1SgMWRs", API_KEY, API_SECRET);
            // SignUp(api);
            CheckIn("GM1056376", api);

And in the call of the checkin function of the api is:

string reply = string.Empty;
        reply = api.checkin(customerId, true);
        ResponseData responseData = JsonConvert.DeserializeObject<ResponseData>(reply);