<form method="post" enctype="text/plain" name="myForm" action="">
         <textarea name="mail" rows="7" cols="15" />
             <input type="submit" value="Send!"
onklick="OnSendForm(<xsl:value-of select="/details/address" /> )" />

You can't use XML markup inside an XML attribute. Use an XML parser to
try and parse your stylesheet - it is not valid XML.

This is called an "attribute value template" (AVT). Look it up in
Google or a book.