That documentation doesn't look up-to-date. Grade schemas don't change all that much and what you're looking for has hardly changed at all since 1.95. You don't mention your version so I'll assume you're wise enough not to be using a version unsupported for security fixes. In that case your query would be:
$params = array('userid' => $USER->id, 'itemid' => <the itemid from grade_items you want to display>);
$gradeval = $DB->get_record('grade_grades', $params);
