This article provides the step by step instruction for adding a text effect in a Keynote slide. For example if you have the following slide with the text as shown below and want add some text effect
Select the Text box and click the Inspector icon to display the inspector Window
In the Inspector Window, navigate to Build In tab and this would allow you to specify the Effect with Direction and Delivery options.
You can pick different effects and Direction using the drop downs.
Now you can also specify when you want to start showing this effect i.e after mouse click or automatically after displaying the slide. This can be done by using the Start build feature available under More Options.
Similarly you can specify the Build Out effect using the option available as part of the Inspector Window.