I Solved it this way.
Code:
RestClient client = new RestClient("http://www.newsblur.com");
RestRequest login = new RestRequest("/api/login", Method.POST);
login.AddParameter("username", "xxxxx");
login.AddParameter("password", "xxxxx");
IRestResponse response = client.Execute(login);
CookieContainer cookiecon = new CookieContainer();
if (response.StatusCode == HttpStatusCode.OK)
{
var cookie = response.Cookies.FirstOrDefault();
cookiecon.Add(new Cookie(cookie.Name, cookie.Value, cookie.Path, cookie.Domain));
}
client.CookieContainer = cookiecon;
RestRequest feeds = new RestRequest("/reader/feeds", Method.GET);
IRestResponse<Classes.FeedData> resp = client.Execute<Classes.FeedData>(feeds);