Follow these steps to set up your new Cellular Data account:
1. Tap Settings > Cellular Data > View Account
2. The Cellular Data Account management window loads. Enter information or select an option in the following sections and tap Next when you are finished
– User Information: Enter your first and last name and your phone number.
– Login Information: Enter an email address and a password. This creates an AT&T account, allowing you to log in to the account later to make changes or view your usage statistics.
– Recurring Domestic Plan Options: This allows you to choose the type of plan you want to use.
– Payment & Billing Information: Complete this section with the billing information you’d like to use to pay for your Cellular Data account.
3. The Terms of Service agreement loads. Take a minute to read this agreement. Tap Agree to continue.
4. The Payment Summary loads. This shows you a summary of the account and plan you have selected (Note: You can also add an International data plan on this screen by tapping Add International Data, or you can access your account later to manage your plan options. If you need to use your iPad internationally, you must sign up for the international data plan while in the United States.)
5. Tap Submit to complete the creation of your Cellular Data account. The Congratulations screen loads. This lets you know that your account has been created, and that AT&T is activating your iPad. Tap OK to finish and return to the Settings screen of your iPad Wi-Fi + 3G.
Your iPad will notify you when your Cellular Data connection has been activated.
Apple offers more info, with screenshots, here.