When was the last time you had it flushed? Ford used 2 types of antifreeze, depending on the year. I think yours was the green type... If it looks brown, it's time to get it flushed.
That being said, if there is fluid in the expansion tank, then just add more to it. If the tank is low or empty, wait until the truck has cooled down and then open the radiator cap and add until it's full, and then add to the tank up to the cold fill line.
Just adding some fresh antifreeze though, is not going to do much if it needs a good flush...