they unlock at certain milestone (I couldn't remember when exactly). For Dangers in the dark the next pack is from Mining, I think it unlocks around 145m, and for Feeding the World the pack is from Horde, after zone 75 iirc.
I think you have to buy the pack once in order to remove the lock from blank cards and see the odds. Dangers in the dark need cards from "Danger Zone" pack. For feeding the world I have no idea, I also have the last card locked.
