Re: [Resolved] Buddypress with WPMU
=== 2. I setup two sites (the main site and a testing site) using sub directories. ===
BuddyPress default blog when you activate it is in main site http://yoursite.com.
Therefore, if you installed WP 3.1 in domain root i.e. http://yoursite.com/, created a network, created a subsite http://yoursite.com/testing, and you want to test BP in this subsite, you need to follow this instruction https://codex.buddypress.org/getting-started/install-buddypress-on-a-secondary-blog/