I am trying to light up my new iPad 3 with the AT&T 4G service. I'm not quite sure how it all works, and not sure of all the steps that I should do. In settings under Cellular Data, there is a window that says "Enter PIN to lock SIM". Is this something that has to be done, or is this an option? Where do I find the SIM's number? Is there anywhere I can go to give me step by step instructions on how to start the 4G service, and how to stop the service correctly at the end of the month cycle? Thanks!!