It sounds like you are trying to use BuddyPress with a version of WordPress less than 3.0. For a few months now, BuddyPress has required WP 3.0 or greater.
In your FTP client, navigate to the wp-content/plugins folder. Delete the buddypress folder. Then go to yoursite.com/wp-admin, and things should be back to life. You will have to upgrade to the latest WP before attempting to install BuddyPress again.