To purchase data for a day, you need to own a slate with cellular connectivity. Head over to buyasession.att.com using the web browser on your tablet. There is a four step process that you need to complete in order to purchase the data. Your device must have an AT&T SIM card.
The Apple iPad, when originally launched in 2010, had an option on the setting page that allowed you to purchase unlimited data on a month-to-month basis for $29.99. While this option is still available, it depends on what the carrier is offering. Some will charge iPad users a prorated fee depending on the number of days in a month that the plan is in use.
source: Buyasession via AppleInsider