I guess the easiest way to enable Latex in bbpress is to use Mathjax.
Put the following code into the block of the header.php in the theme. This will load the latest version of MathJax from the distributed server, and configure it to recognize mathematics in both TeX and MathML notation. For details, refer http://docs.mathjax.org/en/latest/start.html#mathjax-cdn
I do not why the script above is not displayed.
