HT5704: About the security content of iOS 6.1.3Learn about About the security content of iOS 6.1.3
In order to enable 3G on your iPad, you would have to have the Wi-Fi + Cellular model iPad 2. If your iPad is a cellular model, you can go to Settings and Cellular will be an option there. If it does not have that option, then you would not be able to use 3G on your iPad and that would mean your iPad is not a cellular model.
Also, you must be subscribed to a cellular data plan specifically for the iPad through companies like AT&T, Sprint, Verizon, etc.
Here's an articel about this for you http://support.apple.com/kb/HT4203
If you are positive that you have a Wi-Fi plus cellular iPad go to Settings.
The fourth item from the top is Cellular Data.
Turn on Cellular Data and set up an account there with a service provider.