Here is how I did it.

1) ===========================================
in templates_c/#######posting.tpl.inc.php around line #370 added this:

<input class="bbcode-button" type="button" name="youtube" value="<?php echo $this->_config[0]['vars']['bbcode_youtube_marking']; ?>
" title="<?php echo $this->_config[0]['vars']['bbcode_youtube_title']; ?>
" onclick="bbcode('text','youtube');" /><br />

.....and same file line #510 added this:

<?php echo $this->_config[0]['vars']['bbcode_youtube_title']; ?>

==============================================

2) ==============================================
in template_c/########english.lang.cposting.php around line #48 added this:

  'bbcode_youtube_marking' => 'Youtube',
  'bbcode_youtube_title' => 'Add Youtube Video: [youtube]ae73jju9[/youtube]',

==============================================

3) ===============================================
in includes/functions.inc.php around line #598 added this:

 $bbcode->addCode ('youtube', 'simple_replace', null, array ('start_tag' => '<embed src="http://www.youtube.com/v/', 'end_tag' => '&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed>'), 'inline', array ('listitem', 'block', 'inline', 'link', 'quote', 'pre', 'monospace'), array ());

===============================================