Adding the 3G connection is pretty much straighfoward.
1.Add a new interfce: Go to Network -> Interfaces -> Add a new interface
2. Enter the name of interface and the protocol of the new interface (UMTS/GPRS/EV-DO)
3. Time to configure your connection. On general setup tab enter your service provider data. (Here is my configuration)
4. !!!IMPORTANT!!! Add your new configuration to WAN firewall rule, otherwise you wont get any internet through your lan ports and wifi.
In the end your new interface should look something like this: