Re: Your currently active theme is not BuddyPress enabled.
The default BP themes are initially located in the /bp-themes/ directory. Although you do not technically have to move them from that location before activating BuddyPress, you will not have access to them until you move the /bp-default/ and /bp-sn-parent/ folders into the /wp-content/themes/ directory.
Once those two theme folders are moved to the proper place, you then need to log into WPMU as Site Admin and do step 5 under “Installing BuddyPress” here:
Always read the readme.txt files that come with both WPMU and BuddyPress. All the basic information you need to successfully install them and get them working can be found there. More detailed information can be found on the BuddyPress Codex–like the link I provided above.