Make a layout panel the active element
On the artboard or under Objects and Timeline in the Interaction panel in Microsoft Expression Blend, double-click the layout panel that you want to make active. Notice that a yellow highlight appears around the panel, to indicate that it is the active element.
Right-clicking any element on the artboard and pointing to Set Current Selection will provide a list of all elements, in Z order (the order in which the elements are nested), that are under the location that you clicked. From there, you can select any element in the list and then draw elements into it, without necessarily making it the active element.
In the same way that you can make a layout panel the active element in Expression Blend, you can also make controls or other elements active. This action is common when you are adding child elements to an element, such as when you add items to a list box.