Go forward a little ways until the ceiling above opens up. Then you can grapple onto a ledge. Run forward and at the end you'll find a vent. Climb inside and it will bring you around to the giant middle room and your first real Stalker challenge.
There are five enemies in the area. The easiest to take out will probably be the front most two. One of them will be facing the front door and not turn and along the same wall and up some stairs will be another guy facing a different door way. Just wait until they are alone and drop down for a silent takedown. Once you've got those two, there are two more that circle around either side of the room. You may have to explosive gel the glass first but you can actually do an inverted takedown through the glass. Just have patience and wait for him to go right underneath. Take one down and the other two guards will probably go to investigate. Wait until they come out from under there. When they are finally out, select your baterang and use it to hit one guard and immediately glide kick the other. You should knock down both enemies, then just do a ground takedown on both to finish them off.
After a brief talk, head back to the entrance of the building. You'll be trapped and download the unlock codes you need. Drop through the grate in the floor and head into the nearby room. Climb back out and use detective mode to find the unlock panel on the wall. Select your Cryptographic Sequencer. Target the panel and shoot (RT/R2) to start the real hacking minigame.
You want to rotate the sticks to clear all the letters so you're only left with one letter per column. The left stick controls the left bunch of letters while the right stick controls the right bunch of letters. Rotate the left stick slowly until the first column resolves into a single letter. Continue rotating and soon the other letters should fall into place, one by one. Once you have gotten the left group, hold the analog stick there and go through the same process with the right analog stick and word. Once you have both words, the doors should unlock. Head back outside. To get out, you'll have to open the shutter and slide under like you did before.
