If you're confused as to how you unlock more story missions, in survival mode you'll notice little icons on your minimap, I don't know what the icon is supposed to be but it looks kinda like a USB drive or a cell phone. Once you're in view of them they'll appear as little presents.
After you've collected them all you have to play, and finish that mission before more of them will spawn. Once you beat the mission there should be more to collect once you start another survival game.