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