If you can't find it the dealer was supposed to give you a card with the code on it. I suggest you go back to the dealer or call them and have the stock number the dealer had on the truck available. I bought my truck about a moth ago and the dealer handed me a little plastic card that had the manufacturer's code on it.
I just got through this ordeal with my 2002 Explorer. Bye the way thanks everyone for there great ideas. But, it is located on the passengers side on mine. I had that whole truck torn apart on the drivers side and could not find it. Then a guy at work said it's buy the rear wiper fill. Which is on passenger side (on mine). Sure enough five minutes later I could open my door with the key pad!! It is a grey box about 6 inches long by maybe 4 inches wide. It has a 5 digit number stamped on a white label. That's it. It's just behind the door jam of the back passenger door. Just pop the back corner molding off, the molding where the back seat belt is, the molding back by the bumper(runs from side to side of the truck), then pop big pass. back panel loose(don't have to take it out) look in from the top just behind the back seat and you'll see it. Good luck.