I'm authenticating with an OAuth token, but I don't think it should be any different. Have you tried something like this?
var twitter = new LinqToTwitter.TwitterContext(auth);
var tweets = twitter.Status
.Where(t => t.Type == LinqToTwitter.StatusType.User && t.ID == "Microsoft")
.OrderByDescending(t => t.CreatedAt)
.Take(50);