Just to be sure, you are asking how to add a shadow to a building to make it look 3D, correct?
There are a couple of ways you can do this. The easiest is to use Cartographic Representation and you add in a second polygon layer with the Move Effect. The amount you want to move the layer depends on the scale. For example I tend to move the building footprints x = 2, y = -2. You will then have to colour the two polygon layers different colours (black or a grey colour for the moved layer).
You can also create a copy of your building footprints feature class and then during an edit session, select all of the features and use the Move tool to move all of the features (e.g. x=2, y= -2). In the Table of Contents you will have to add both the building footprints layer and the building footprints layer that you just moved.