Assuming you don't have the original security card that came with the manual which had the 5 digit number on it then you will need to remove the back of the back seat, pull back the liner on the driver's side of the back of the cab and read the number off of the control unit. I just did this 7 days ago on my 08 Screw after buying it used.
Or, you can take it to the dealer and have them plug into the computer and read the code, there may be a charge for this however.
Directions for entering your own code are in the manual.
How to remove the back seat back (Also shows a pic of the control unit with the number on it): http://www.ford-trucks.com/article/i...2008_F150.html
Some more helpful tips: http://www.ford-trucks.com/forums/59...rear-seat.html
I didn't remove the seatbelt bolt, just pulled slack in it to get the seat out from under it.
Hope that helps.