If you search google it will give you locations to look for the code. There are a few locations behind panels, under the dash etc. The locations are different in different years. I bought mine used and the master code was written in the manual. Ford dealers are able to pull the code from the computer. If your a talker or have a friendly dealer they might do it for free. With the master code you can make a resetable personal code as out lined in the book and on the net.
2006 f250 4x4
Some tuning, gauges and other assorted non factory dodads
Lots of people have this problem and I was one of them. My local dealer, with a straight face, quoted $98 to get the code out of the computer. That made me so mad, I decided to find out how to do it myself and tell as many people as I can how they can do it as well. I'm not a Ford mechanic, but I have a 2004 F-350 and I can tell you how I did it. The RAP module was mounted on the fire wall behind the radio. I suspect the easiest way to read the code is to remove the radio. If you can't get it out, remove the fuse panel located below the steering column. With a bright flashlight look to the right and up behind the radio and you should see at least part of a large white label. It will have a large bar code on it as well as the words "Canada" and "FCC" among other stuff. The factory code is the bold five digit number located to one side. This was not easy to see. I had to reach up under the panel and push a wiring loom out of the way while my wife looked for the number, but it was easily worth $98.
OH! and after I found mine I found it written in black marker on the door pillar ,, they thru a few extra #'s to confuse folks but it was a good idea! there is also a sequence of codes available that will get anyone into these trucks in 20 minutes . I have my key less viper and I don't have the keypad on the door able to disarm the alarm