you don't need to add the form element:
<textarea name="SpecialInstruction" cols="50" rows="5"><?php echo $_SESSION['SpecialInstruction']; ?></textarea>
only the session binding:
<?php echo $_SESSION['SpecialInstruction']; ?>
after adding the session binding, save the email message, then click ok in AUniversal Email to except the changes and save and upload the files to the server.