You will unlock this after completing the first 7 levels, not including boot camp. The first chapter is pretty easy though. You shouldn't have trouble with it. If you are having trouble, make sure you are upgrading your troops when you can, and don't be afraid to use consumables. The rockets in this game can take out a vehicle in one shot. I find that one of the most useful things in the game is the Intel pack. I recommend buying it every level, except for the first level, since the map is so small, survival levels, and levels which you are mounted on a vehicle.