[Adam Harley (Kawauso)]

jQuery is already bundled with bbPress (and WordPress), so you can just use

<?php bb_enqueue_script('jquery'); ?>

in your theme’s functions.php and it’ll automatically be included in your header.

Same applies for WordPress, but using wp_enqueue_script instead…

But to answer your original question use <?php bb_active_theme_uri(); ?> to output the template directory or <?php get_bb_active_theme_uri(); ?> to return it as a variable.