ok, save this file as bbpress.php into the root of your theme
ie
wp-content/themes/%yourthemename%/bbpress.php
where %yourthemename% is the name of your theme !! 
Then add a line above the last, so that it now reads
<? get_sidebar(); ?>
<?php get_footer(); ?>bbpress will now use that page as the one for bbpress, and should display a sidebar.
‘tweaks’ should tghen sue this fro your forum sidebar.
Come back if any of that doesn’t work, or you need more explanation