Before you could use the 3G/4G network on your new ipad 3, you are required to have a SIM card inserted into the device. However, you may that it is difficult to have the SIM tray opened, and you are afraid to break it. This you have a problem with opening the SIM tray, try the following:
1. First turn off the iPad. Press and hold on the Sleep/Wake button until you see the red slider appears, then slide the slider across to turn it off.
2. Next have your iPad face down on a table.
3. Now use a SIM Eject Tool or a small paper clip, and insert it into the small hole next to the SIM tray. You will need to have pin/tool at about 45 degrees angle to horizontal, i.e. roughly perpendicular to the case where the hole is. You need to push it quiet hard to pop the tray out.
4. Now place SIM card on the tray and push it back into the device.
5. Turn on your iPad and you see the Cellular signal.