BuddyPress.org

What do you mean by this?
“My WP v.2.91 (subdomain WP MU) is integrated with WP MU Buddypress”

If you have BuddyPress installed on the WPMU subdomain and you’re trying to use BP code on your single WP install, you can’t as BP doesn’t know about your single WP install.

If you can, I would suggest using your conditional on you WPMU subdomain only.

“Big picture”-wise, you’ll need to think about upgrading to WP 3.0 and think about how you’re going to manage your single WP site and your WPMU network.