"Recent Visitors" is what the "who's online" turned into when you upgraded. Check the miscellaneous tab in your control panel and set those options for recent visitors, avatars, polls, and smilies (graemlins).
Also, under the maintenance tab, make sure you've used the "upgrade your ubb" link. After that, use the other links in there (rebuild forum stats, rebuild search index, remember member stats, and rebuild member post histories). You'll need to do that to finish the upgrade process.