[mrmammoth]

Проблема решена:

<script type='text/javascript'>
        function doAddContainer() {
            obj = {
                name: "tile002",
                thumbnail: "images/widget_scroll.png",
                content: "tile001",
                url: "tiles/blank.php",
                size: "4x2",
                theme: "orange",
                link: ""
            };
            objobj = { obj: obj };
            tile = {
                size: "4",
                theme: "container2",
                name: "container2",
                tiles: objobj

            };

            var cc = document.getElementById('plugin_textarea_string');
            tiles = cc.value;
            tile_containers = unserialize(tiles);
            tile_containers[tile_containers.length] = tile;

            document.getElementById('plugin_textarea_string').value = serialize(tile_containers);


        }
    </script>