It’s new in the Bootstrap 3.0.1 release, so make sure you have the latest (10/29)…
Demo: http://bootply.com/91632
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
<div class="row">
<div class="center-block" style="width:200px;background-color:#ccc;">...</div>
</div>