Ohh, interesting question! I’ve never seen it done on a live site, but I set it up a few weeks on my test installation.
If you want to use the Multi-level Navigation WordPress plugin with your bbPress install, you just need to do a deep integration with WordPress. There are instructions on http://bbPress.org/ on how to do this, but basically it just involves adding require(‘pathto/wp-blog-header.php’) to the bb-config.php file in bbPress. Then you can just follow the same steps with your bbPress theme as you would with a WordPress theme.
If you aren’t wanting to use the WordPress plugin, then the process is identical to any other menu. Just add the #suckerfishnav ID to any unordered list in bbPress, add the CSS from the generator and a menu should appear instead of a boring list of nested links.