Can't change inspectable object


im currently working on a flash component (first time). Everythings find - but -
I have an instance of my component dragged onto the stage.
Played around with changing the different properties i made inspectable - so here it comes:

The property (typed object) i would like to change can not be edited in the IDE.
I have no plus icon to add a new name-value pair to the list - any suggestions?

here is my code with the inspectable meta tag:

    protected var _data:Object = {};
    public function get data():Object { return _data; }
    [Inspectable(name='data', type='Object')]
    public function set data(value:Object):void {
        _data = value;

and a screenshot of my empty panel where i would like to add some data:

Would be glad if someone could help me with this.