The service objects can be found in the panel on the right side of the screen inside the app builder.
Start by drag and dropping the object onto the canvas or simply click to add it automatically.
Their color representation is purple:
Service Objects are enabling you to access external services like Voice, Music, SMS or many more. Each service has to properly be configured in order to work with your app. You can install Services from the Store.
To find out more about certain objects please refer to the Object Reference.