To select a part, move the cursor over the part and click. A dotted grey rectangle appears around the selected part and any rotation or resizing controls are also shown. To de-select a part, click on the scene background.
You can also select a part by clicking on the drag area of the part's toolbar. This can be very useful for some parts, such as Button and Animation. You cannot select these by clicking because the clicking action is reserved for pressing the button or playing the animation.