Hello Mom2niles,
Welcome to the community!
Are you referring to using one of the 5 Extra Hours of data? If so, you should be able to initiate a session using the My Account app. Once the app is opened, the option to start a data session is located right under the Usage / Phone / Plan tabs. There are some limitations, such as the hours of data are not available when roaming. You can view other information regarding those hours of data here.
On the other hand, if you are referring adding data to your plan if you've exceeded your monthly allotment of data, you should also be able to add data using the My Account app (see here, under data).
Hope this helps 😀
Cheers
