To make animated elements appear, switch to present mode and then just click on the forward command (mouse click, play arrows on the screen, or others) as you always do for presenting. The animated elements will appear as you move forward through your steps. Take a look at the following screenshot:
The fade-in effect is activated only in present mode. When in present mode, animated elements appear as you move forward in your presentation, working your way through frames and animations on your path.