google dfp api integration

hi guys,
i am facing a little probelm in retriving the refresh token, using google's OAuth 2.0 
https://code.google.com/p/google-api-dfp-dotnet/wiki/UsingOAuth#Adding_OAuth2.0_support_to_your_ASP.NET_application
oAuth.FetchAccessAndRefreshTokens(Request.Params["code"]); 
                string RefreshToken = oAuth.RefreshToken;                 //value ""
                string AccessToken = oAuth.AccessToken;                 //getting value
  &nb ...