BuddyPress.org

You have to visit the back end of wpmu as site admin to upgrade your tables to RC2. What it does is in sequence:

1) Create a new site wide user activity table

2) Move all the individual user activity table entries into the new site wide activity table

3) Remove all the individual user activity tables

4) Set a flag that this has been done in site options (meta) called ‘bp-activity-db-merge’

bp-activity.php – bp_activity_install() Jeff.

Did you visit the wpmu back end as site admin?