Okie dokie y'all so to get this achievement, you can't miss it! Its story related!
But Im going to give you the steps for you to know what to do to get through the P.D.
1. A cutscene will play and you will be playing as Carlos
Cue another cut scene
2. Now you can beat up brad if you want, it will help you get another achievement if you kill him and take his key card.
3. Follow your buddy into the R.P.D. Grab the items you need or want and wait for his signal
cue another cut scene
4. He will lift open the grate, your more than welcome to walk around there aren't any zombies yet to grab things. When you get into the first room after the grate there will be a box to your left, if you killed brad than use the STARS keycard on that box and yayy free stuff!
5. Continue to the next door, cutscene, than move down the hallway depending on what difficulty you play on depends on what spawns, it could be a zombie or a licker :) When you get closer to the double doors another zombie will pop out.
6. Go through the double doors, as you see from the last game you cant jump yet so make your way through the room, again if there is stuff and you wanna pick it up go for it!
Cue small cutscene of carlos and friend talking
7. Unlock the door there is going to be a mess of zombies out in this room, plus one or two of them will pop out from doors, kill them all if you can. If you want to run past them go for it! From the room you exited you want to run straight and to the left down a long corridor where you can see stairs at the end, keep going! Now go up the stairs, you will reach the second floor, great! Keep going up the stiars!
8. There is going to be a zombie on the third floor, kill it and continue up to the fourth floor there is going to be a wooden box where a key lies, if you wanna kill that zombie and you know the locker combo great if not (DCM)
9 Now you are gonna wanna go back down to the base floor, where all the zombies lived, but now if you didn't kill all the zombies you have a licker that has spawned. If you want to avoid everything cool beans if not kill it and go to the first door on your right from the corridor and use the key!
10. Enter the storage room, there is a sleeping zombie in here kill it and go to the control panel and type in 104 and if you want the other stuffs you now have a battery :D
11. Now head back up to the second floor where the janitor stuff was, go into the door and there will be a detonator on the wall in the bathroom, again there is stuff in here if you want to grab it. Pick up the detonator (HANS THE DETONATOR) and combine the two now a cutscene will play
12 There is once again a mess of zombies kill them or avoid them if you can, once you clear everything out and leave this area a licker will spawn in the corridor, if you want to run it you can it will be the first door on your left!
Cue another cutscene, look around for anything you want and than exit.
Cue another cutscene and boom the achievement should pop!
Good Luck hope this helps!