You have to select the placeholder-layer in the timeline. Holt alt (Windows) or option (mac) on your keyboard and drag and drop your footage on the selected placeholder-layer.
Adobe Effects Help:
To replace the source footage item for a layer with another footage item, without affecting changes made to the layer properties, select the layer and then Alt-drag (Windows) or Option-drag (Mac OS) the new footage item onto the layer in the Timeline panel. To replace all uses of a footage item with a new footage item, select the currently used footage item and then Alt-drag (Windows) or Option-drag (Mac OS) the new footage item onto it in the Project panel. All layers that refer to the current footage item as their source will use the new footage item as their source.