I figured out how to install Wasabi WP Related Post plugin on WordPress MU.
You will need to run the MySQL script manually. Instead of…
ALTER TABLE `wp_posts` ADD FULLTEXT `post_related` (
You can replace ‘wp_posts’ to the database table used by that particular WordPress MU blog, for example, ‘wp_18_posts’.
I have done it for this blog. I used the Peter Bowler’s version of Related Post. It works.