Hi jmontano,
I checked it's working refer below sample code.
When i remove class then message is now showing.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<table border="0" cellpadding="0" cellspacing="0">
<tr>
<td>
</td>
<td>
Please fill the following Form
</td>
</tr>
<tr>
<td>
</td>
<td>
</td>
</tr>
<tr>
<td>
City:
</td>
<td>
<asp:DropDownList ID="ddlCities" runat="server" CssClass="form-control selectpicker validate[required]">
<asp:ListItem Text="Please Select" Value="" />
<asp:ListItem Text="Mumbai" Value="1" />
<asp:ListItem Text="Delhi" Value="2" />
<asp:ListItem Text="Kolkatta" Value="3" />
<asp:ListItem Text="Chennai" Value="4" />
</asp:DropDownList>
</td>
</tr>
<tr>
<td>
</td>
<td>
<asp:Button Text="Submit" runat="server" />
</td>
</tr>
</table>
</form>
<link href="ValidationEngine.css" rel="stylesheet" type="text/css" />
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css" />
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript" src="http://cdn.ucb.org.br/Scripts/formValidator/js/languages/jquery.validationEngine-en.js"
charset="utf-8"></script>
<script type="text/javascript" src="http://cdn.ucb.org.br/Scripts/formValidator/js/jquery.validationEngine.js"
charset="utf-8"></script>
<script type="text/javascript">
$(function () {
$('#<%=ddlCities.ClientID %>').removeClass("validate[required]");
$("#form1").validationEngine('attach', { promptPosition: "topRight" });
});
</script>
</body>
</html>
Screenshot
