Are you trying to add tabs inside your FB application canvas page?
WPBook renders that page as an iFrame, which means it isn't using FBML on the canvas pages. (More accurately, there is javascript call which renders certain FBML after page load - see these lines in wpbook/theme/index.php, toward the bottom:
<script type="text/javascript">
FB_RequireFeatures(["XFBML"],function() {
FB.Facebook.init('<?php echo $api_key; ?>',
'<?php echo $receiver_url; ?>',
null);
FB.XFBML.parse(document.getElementById('addProfileButton'));
});
</script>
This takes the div with an id of 'addProfileButton' and parses the XFBML in it. You could probably do the same for any other div in which you want to put FBML, so long as it is part of what FB supports under the XFBML standard.