In order to run the application, click the Run 'app' (^R) button at the top-right side of the IDE window and select a deployment target. The HelloApp will be built, installed, and launched on the deployment target:

You may use one of the available prepackaged virtual devices or create a custom virtual device to use as the deployment target.  You may also decide to connect a physical Android device to your computer via USB and select it as your target. The choice is up to you. After selecting a deployment device, click OK to build and run the application. 

Upon launching the application, our created layout is rendered:

 

When CLICK ME! is clicked, Hello World! is shown to the user:

Subsequent clicks of the CLICK ME! button display a toast message with the text I have been clicked!: