require_once(dirname(__FILE__) . '/../wp-load.php'); (PS my forums is set up in a forums directory in the root of my site.
You may need to change the path to your wp-load.php file)

Next create a functions.php file for your theme if not already there. Then add this line of code to the functions.php file.

add_action( 'bb_foot', 'bp_core_admin_bar');

Now create your menus and add them to the bar.

PS because the buddypress plugin is not activated on a forum, it will load the buddybar with nothing in it. If you are going to copy the default buddypress functions then make sure to change the function name or you will get and error.

Alternatively you could try include the bp-core-adminbar.php file inside buddypress/bp-core if you do not want a blank bar.

Add the following line to your functions.php file to disable the BuddyBar

define ('BP_DISABLE_ADMIN_BAR', false);