When you say you want to insert a line break, do you mean a new line in the generated XML, or a newline in the text as displayed by a browser?
In the former case, insert <xsl:text>#x0a;</xsl:text>. In the second case, insert <br/>.
If inserting <br/> didn't work, then you need to explain how you tried to do it so we can show you where you went wrong.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference