Cell Phone Unlocking Problems!?

i just got a LG CHOCOLATE from mexico (one that my cuzin had) and her company was Telcel. I tryed putting in the SIM card but it says it dosn't work. So then i called At&t and they told me i had to UNlock it? i need help unlocking it?! Here is the Phone (not a usual chocolate)


http://www.simoncells.com/images/LG_Chocolate.jpg