I believe the linebreak is the
<w:br xmlns:w="#unknown" />
tag and you can use
Environment.NewLine
to write a line to the text file.
Note that plain text files don't behave in the same way as Word files so you can't do much in the way of formatting.
The following CodeProject articles may also help
Show Word File in WPF[
^] and
Find Text in Word Documents[
^]