If you are using solution from http://blogs.msdn.com/infopath/archive/2005/03/04/385577.aspx. You need to make sure that your textbox is formatted to be able to display paragraph breaks. To do this double-click on the text box control, go to the Display tab, and check the checkbox labeled “Allow paragraph breaks”.
After this, your code should work, but if it still doesn’t try using “\n” instead of the System.Environment.Newline.
There is another way to do that.