It looks like you already have the drilldown functionality set up correctly so you just need to make sure the items involved have certain attributes set appropriately when the report loads.
To get everything expanded by default, you need to make sure that the detail row's Hidden attribute is set to False, i.e. show when the report loads, and the toggle item, i.e. the Loc group textbox, has its InitialToggleState set to True, i.e. make sure the textbox has a (-) sign when the report loads.