I believe that Dreamhost’s “One Click Install” of WordPress has the Page Revisions turned off by default. This must be something new, because my older sites have it enabled but ones I installed recently do not.
The fix is to access your site via FTP, edit the wp-config.php file, and look for this line:
define('WP_POST_REVISIONS', false);
Either delete or comment out that line, then save and re-upload the file.