Here are a couple of ways I've tried. I am using version 0.9.0.3. This is the page that Twitter calls back after authorization. I've confirmed that twitter is sending me back oauth_token and oauth_verifier values. Each time, the newCredentials object is null.
var applicationCredentials = CredentialsCreator.GenerateApplicationCredentials("MyTwitterConsumerKey", "MyTwitterConsumerSecret");
var newCredentials = CredentialsCreator.GetCredentialsFromCallbackURL(HttpContext.Current.Request.Url.AbsoluteUri, applicationCredentials);
var oauthVerifier = string.Empty + Request.QueryString["oauth_verifier"];
var applicationCredentials = CredentialsCreator.GenerateApplicationCredentials("MyTwitterConsumerKey", "MyTwitterConsumerSecret");
var newCredentials = CredentialsCreator.GetCredentialsFromVerifierCode(oauthVerifier, applicationCredentials);