you need to click your own cities or allies city to start dispatched. If you still don’t work.. here the solution:
1. Try reading FAQ in main page of dawn of nations
2.You forget to click ON YOUR CITY or ALLIANCE’s CITY and click transport ( no matter anything you click onto YOUR city)
3. You forget to filling up a truck or you forget the main dispatching phase using trucks.
still don’t work = either you don’t GET IT or the technician problem.