Can't show plus icon to show a hidden column as child

following conversations i found in blog, and using this one:
http://live.datatables.net/wozojore/738/edit
i tried to make a page:
http://mps-roditi-com.stackstaging.com/announcements/ForTest.html
showing only two columns instead of three. The idea is to show the third one using plus sign as a child.
i used

responsive: true,
            ajax: '../mpsDatatables/php/table.announcements.php',
            columns: [{
                    "data": "ptitle"
                },
                {
                    "data": "pmessage",
                    visible: false,
                    className: 'none'
                },
                {
                    "data": "pdate"
                }
            ],

but it's not working.
Am i missing something?
Thanks in advance