BuddyPress.org

You need to ask that question on the Premium WPMU site. It is a 3rd-party project not supported by Automattic.

If you want a free multiple database engine, you can use HyperDB. This site runs on HyperDB. WordPress.com runs a highly-modified version of HyperDB.

Here are a series of articles written by Trent Adams, one of the BuddyPress moderators, that detail how to configure HyperDB:

http://trentadams.com/2009/04/18/hyperdb-explained-part-1/

http://trentadams.com/2009/05/08/hyperdb-explained-part-2/