Because of the complications that go along with upgrading a live database, the Dreamhost One-Click Installer doesn't normally modify the database if you perform a One-Click Upgrade. In this case, it looks like MediaWiki made some changes to their database schema, which is why you've got this problem now. Fortunately, the fix looks pretty easy.
According to the MediaWiki upgrade guide, all you need to do is log in through the shell and run the maintenance/upgrade.php script. Just keep in mind that on Dreamhost, the default PHP version in your path is PHP4, and MediaWiki needs PHP5, you'll need to type the full path to the PHP5 interpreter to run the script. As such, anywhere the guide tells you to type
you should type