Taken from: The WordPress MU Manual
bbPress is forum software created by WordPress.org that easily integrates with your WMPU site.
Setting up MySQL Database and User for your bbPress install
Before you can install bbPress you first need to create a MySQL database and user on your domain using your cPanel.
1. Log into cPanel using your account details supplied by your hosting company.
NOTE: Appearance of cPanel does vary slightly depending on what theme your hosting company uses.
2. Scroll down to the Database section of cPanel and click on MySQL Databases
3. Create a database by entering the name bbpress and clicking create database
4. Once the database has been created click on the Go Back link.
Note: the name of your database and user are both given the default prefix of your domain name. This is to ensure it has a unique name from other databases when it is stored on your host’s servers.
5. Now add your user (wpmu) you created for your WMPU database to your bbpress database by scrolling down to Add User to Database and click on Add.
6. On the Manage User Privileges page select All Privileges.
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August 4, 2009 
Thanks for the step by step!