I have been struggling with how to integrate a site-wide search on a WordPress MU site for a few days. I tried One Search, wpSearchMu, and a couple of others, but I could not get any of them to work. Finally, I installed Donncha‘s WordPress MU Sitewide Tags Pages plugin.
The install process is simple (just click the check box at the bottom of your “Site Admin”–>”Options” page and then click the “Update Options” button). From there, I had to add the following code to my template files in order to pull the search from the “site wide tags” blog:
<?php switch_to_blog(12); ?> <?php include (TEMPLATEPATH . '/searchform.php'); ?> <?php restore_current_blog(); ?>