1. download the latest version of TinyMCE , I use version tinymce_2_1_0
2. unzip file and upload "jscripts" folder to your server/bbpress/my-plugins/, so link will like this (http://example.com/bbpress/my-plugins/jscripts)
3. add code to the bbpress templates header file (http://example.com/bbpress/bb-templates/kakumei/header.php),
ADD CODE before '/head' of header.php
<script language="javascript" type="text/javascript" src="http://example.com/bbpress/my-plugins/jscripts/tiny_mce/tiny_mce.js"></script>
<script language="javascript" type="text/javascript">
tinyMCE.init({
mode : "textareas"
});
</script>
4. Done! you can check it out when Post content in your bbpress. :)
see demo capture: http://img147.imageshack.us/img147/5363/tinymceao7.jpg
p/s: I need someone pitch in and help me how to hack bbpress to support more allowed markup like <u> <tr> <td> <font color> <font size>... etc