Service components are invoked via events. Here is how to invoke a Data Source on button click:

1. Select a button component, open Events view
2. Click Add Event > Click. Click event will appear in the list
3. Click + button (right hand side) and select Invoke Service action. 
4. From the drop down list, select data source to invoke
5. Click OK.
   

When testing the app, when this button is clicked, the associated data source (service) will be invoked. You can use any other event/action combination to invoke the data source. 

How to invoke on page load