Can't get an array using attribute selectors
function toggleUserTypeRows(){
    var selected = $('#userTypeDropDown').val();
    switch (selected){
        case 'User':
            var $rows = $('tr[userType="User"]');
            $rows.show();
            $('tr[userType="Admin"]').hide();
            break;

        case 'Admin':
            var $rows = $('tr[userType="Admin"]');
            $rows.show();
            $('tr[userType="User"]').hide();
            break;
        default:
            var $rows = $('#tableUsers tr');
            $rows.show();
    }
}

<table>
    <tr userType="Admin"><td>Row 1</td></tr>
    <tr userType="User"><td>Row 2</td></tr>
    <tr userType="Admin"><td>Row 3</td></tr>
    <tr userType="User"><td>Row 4</td></tr>
 </table>


<select>
    <option value="User">User</option>
    <option value="Admin">Admin</option>
</select>

<script type="text/javascript">

$('select').change(function(e){
    var userType = $(this).val();
    $('table tr').show();
    $('table tr[userType="'+ userType +'"]').hide();
});

</script>

case 'User':
        var $rows = $('tr').filter(function(){
             return $(this).attr('userType') == 'User';
        });
        ...