Iain
There is a couple different ways to do this. If all your upper cabinets are at the same height, all the tops line up, then you can right click and select items to install. Install item, then click on moldings. You can now select the molding and give it a length. Another way is to add the molding to the cabinet when you design it int he cabinet editor. Then when you save this cabinet and load it in the room it will have the molding on it. You will add the molding to the cabinets in the molding editor. If you have any questions feel free to call me.
Start with a new cabinet, remove everything but the top then add crown. Save the cabiinet and add it to your room. This gives you a sizable crown that can be added easily to any room.