This achievement was broken for me when i did my first play through. To get this to pop i had to start a new game. i deleted my old save data and started fresh.
To unlock the bunker you need to get 75 stars. Play missions to get stars. Once you finish the mission that put you at 75 stars it should pop a couple seconds later. If not i would try going into the level and if it still doesn't pop i would just start a new game