Download, install and activate the bbPress Integration plugin for WordPress. Go to the settings pages and enter your bbPress URL and select your WordPress installation. I haven’t tested this for MU so I don’t know if it will work. Just select WordPress for now.

If you have your plugins directory in a different place that add the direct file path. Otherwise just leave it blank.
Save your settings and the Manual Cookie settings will change. Should look something like this:

define( 'COOKIE_DOMAIN', '.your-domain.com' );
define( 'COOKIEPATH', '/' );

Copy those two lines of code and paste them into your wp-config.php file right below the define('DB_NAME'...) line. Save it.
Paste them into your bb-config.php file as well, right above the // ** MySQL settings - You can get this info from your web host ** // line. Save it.
Now you should be able to log into your bbPress forum and you will automatically be logged into your WordPress install. Doing it the other way should work as well.