New Post: Windows Form App Help

I am not sure I understand your question entirely.
What I understand is that you want to add the name of the user who created the tweet in your Textbox.

If ny understanding of the question is correct then:

var tweet = ((ITweet) progressChangedEventArgs.UserState);
string tweetCreator = tweet.Creator.ScreenName;
string tweetText = tweet.Text;
var message =
tweetsTextBox.Text += String.Format("> {0} : {1}", tweetCreator, tweetText);

Regards,
Linvi