DeWitt manages to open the security gate, thus opening a path that eventually leads to Elizabeths rescue.
Theres only one BioShock Infinite collectible in Chapter 33, because the one that is in the actual office is counted in the previous chapter. Its location is presented in the following collectables guide.
After you open the security gate using the switch inside the Wardens Office you need to go back to the elevator you used to reach The Atrium (the guide assumes you acquired the Voxophone collectible from the office in the previous chapter).
There are several enemies along the way, but the path is shorter because the grate that blocked your path earlier is now unlocked.
Immediately when you enter the elevator grab the BioShock Infinite Wardens Office Voxophone (there are two of them if you didnt picked up the first one at the end of Comstock House chapter).
ENDING IT; Elizabeth
Use the elevator and follow Elizabeths screams to find the Luteces once again. After the cutscene you are transported in The Operating Theater.