How I can add heroes in my trucks?

You can assign heroes by tapping on the barracks buildings in your city or the APC menu that you access by tapping the Truck icon on the top left of the screen. In the APC menu tap the cog icon on the top right of each APC that you have unlocked to assign heroes and troops.

To assign heroes to the APC though you need to unlock each of the 3 hero slots via research. Each APC has its own research tree in the institute, the first item of each unlocks the first hero slot. After that you need to work your way though to reach the items to unlock the other slots.