[@robkk Moderator 11 years, 10 months ago]

see if this helps

https://github.com/robkk/bbPress-Hierarchical-Forum-and-Category-Layout

download the .zip file and upload the loop-forums.php file to your bbpress folder in your child theme.

you create a category by create selecting new forum in the WordPress backend and changing the type to category.

to put forums under each category make sure all the forums you want under a specific category has the parent as that category

the docs should help out alot for learing how to use a child theme for your bbPress installation

Codex