when you start the mission you are given a choice of either go with NATO or to find James who contacted you.
Chose to find james and once you find him he tells you about a truck you must steal and then to head north to the ruins to bring it to miller.
One you get the task assigned to steal the truck, In the task menu there will be "Find Miller" and I thought that maybe I could confront miller without the truck and when I did nothing happened. Then I read the little discription and it said "Return to miller when you have the truck".