To active Cellular Data Services you have to use Cellular Data settings on your iPad with Wi-Fi or 3G connections only. You can change your data plan, turn cellular use on or off, or you can also add a Personal Identification Number (PIN) to lock the micro-SIM card on your iPad for security.Activate, view, or change your cellular data account: Go to Settings > Cellular Data, then tap View Account. Follow the onscreen instructions.

Here, How tosetting Data Cellular and Data Roaming on your iPad :

• Turn Cellular Data on or off : Go to Settings > Cellular Data. If you’ve considered to set your Cellular Data to off position that’s mean, all data services will use only Wi-Fi, including email, web browsing, push notificationsand other services. If you  turned on Cellular Data on your iPad, that’s mean that carrier charges may apply (depending on your packet of your cellular operator). For example, certain features and services such as Messages transfer data, and your use of these features and services could impact charges to your data plan.
• Turn Data Roaming on or off : Go to Settings > Cellular Data. If you don’t wish any charges when you on different network provider, you may Turning off Data Roaming services.