Its automatic and once you are out of data, you would be moved to the add-ons automatically. Same with Intl calling. Data and calling addons don't expire until you use all the limit. You can check the remaing balance of addons in your account.
@Samirbista, login to your account. From the overview tab, click on purchase add-on. From there you will see two add-ons priced at $0. Select both of them for purchase and click buy at the bottom of the page and check out. Go back to the overview tab and you will see it there. Take a look at this post. A screen cap is provided.
