This would be listed under Settings>Cellular Data at the top/left of the screen.
If you do NOT see it,
Do you in fact have a model that supports Wi-Fi AND Cellular? Look on the back of the device for a model number..."Model xxxxx" (xxxxx being a series of letters & numbers) and post that here so we can check if it is indeed a Wi-Fi/cellular model.
Here is Apple's iPad identity page (identify your model by the model number mentioned above here):