From the Account List page, select your Prepaid account and then click on the "Manage Account" button.
That takes you to your dashboard. Scroll down to the Promos section and then click the "Subscribe to a Promo" tab.
You will then see the list of Call, Text, Call and Text, Surf, International, and Roaming promos available for Prepaid.
Decide on the Prepaid promo you want to avail, then click on "Subscribe".
Click on the "Yes" button to proceed with your registration."
A pop-up message will then appear on your screen, saying that you'll receive a SMS confirmation upon successful registration. Click "Ok".