How to use @Action in Mobx + reactjs?

I am confused on how to use @action in my code.

class Items {

  @observable items= [];

  @action addItem() {
    let newItem= new Item();
    items.push(newItem);
  }
}

@observer
class ItemPage extends Component {

  constructor() {
    super();
  }

  render() {
    const {addItem} = this.props.store;
    return (
      <div className="items">
        <input type="button" value="add" onClick={addItem}/>
      </div>
    )
  }
}

const store = new Items();