BuddyPress.org

As r-a-y just pointed out, it is called an external install of bbPress because it is external to BuddyPress. So, there is no secret BP code to make it work. You need to integrate bbPress into WordPress.

Once that is successfully accomplished, you then place a link to your separate, external forums within your BuddyPress site. Now, if you want to make your bbPress site look as if it is part of BP, then you have to theme your bbPress to look like the BuddyPress theme you are using.

That is all that was done on the BP.org site–an external install of bbPress with a theme that matches that of the current site theme so that the two appear integrated. In reality, bbPress is not integrated with BuddyPress at all. The bbPress theme does need a function call or two to make the BP menus work within bbPress, but that is covered in a number of other threads.

Have you seen this thread? Perhaps this will provide more insight on how to integrate bbPress into WordPress: http://www.wpmods.com/integrate-wordpress-bbpress