In the Project Editor, select the Location(s) and System where you want to add the Item(s) in the Drop Zone.
Expand either the Product Explorer, Labor Explorer, or Package Explorer by clicking on the tabs on the left side of the Project Editor interface.
Make your selections in the Item Explorer that you've expanded then click and drag the Item over to the Add section in the Drop Zone. In this example a Package is being added to three Locations at one time: