Here is another answer to this problem. When I passed the input key there, it gave the error "Keyword Lack", so instead of "passe" "\ r \ n" it worked fine.
WatiN.Core.IE ie = new WatiN.Core.IE ():
TextField txtChoices = ie.TextField(Find.ById( "ctl00_ContentPlaceHolder1_TxtChoices" ));
txtChoices.TypeText( "NotBad" );
System.Windows.Forms.SendKeys.SendWait( "\ \" );
txtChoices.AppendText( "VeryGood" );
System.Windows.Forms.SendKeys.SendWait( "\ \" );
txtChoices.AppendText( "" );
vinaykumar