You've just set foot out of the elevator and into Ron Freedman's apartment building after a whole lot of rooting about underground. This should be an easy stroll down the hall to his flat -
- only it isn't, because it turns out you got out of the elevator one floor too early. Prepper says you need to take the air vents to get to Ron Freedman's place. Dang.
Head down the hallway from the elevator and you'll probably see a zombie slip over the barricade at the end of the hall. Kill it, then wait for another to do the same. Take it out similarly. The party room is next to you, but you have to go through the door to your right to get to it. Open the door, lure the zombie inside out into the hall to kill it, then kill the one that's laying on the table in the middle of the room. Check the walls by the table for an air vent you can crawl through.
In the next room you'll find a bathtub and laundry room, and a zombie will crawl in from the opposite room. Take it out, then check the washroom it was in for Handgun Ammo. (In the toilet. Ew.) Remove the barricade on the other door in the room and go out into the hallway to find a door to the party room.
As you discovered from your radar, this room is full of zombies. Oddly enough, though, they are mesmerized by the purple strobe light filling the room, and won't react to you at all. Even hitting them won't quite break them out of their funk - but turning the lights off will. Avoid touching the DJ's laptop and move through the room, killing them however you like, or ignoring them completely. Once you're done admiring the music, head through the door in the back of the room to find another air vent.
Slide through and up the air vent until you hear something scrape behind you. This is a zombie chasing you. Keep moving and you'll find your way out - right into the hands of a SWAT zombie. Despite the urge to kill quickly, you have more than enough time to beat the SWAT zombie with the Cricket Bat before the zombie in the vent shows up. Take them down, then go through the nearby door.
You're now, officially and finally, in Ron Freedman's flat. Unfortunately, it is a twisting maze of computers and boxes, and you can hear sounds of zombies inside. The first zombie is just inside the room; the second is near the back. This allows you to lure them to you one-by-one without trouble. After you've killed them both, check the walls on the left side of the flat for a CCTV junction box. There's nothing else of interest in here, so head to the very back of the flat and check a shelving unit against the wall for a secret door.
Inside you'll find a bunch of Ravens, all dead (one of them is presumably Ron Freedman) and overhear a message from someone you don't know called Sondra. Scan Freedman's desk for a code you can use to open the safe on the wall. Inside is the Studium Contagione, Shotgun Ammo, a Firepower Upgrade, a few pieces of food and several animal corpses you certainly don't want to eat. Don't go for this stuff right away, however, as two zombies will barge in on you, one crawling and one walking. Kill them first, then take everything of value.
Prepper thinks it's time to get out, so you'd best listen. Crawl through the vent one of the zombies came through to the kitchen. Here you'll find Documents on the table and a rubbish chute. Get in the chute and you'll be sent back to the bottom of the apartment, where a zombie is waiting. Kill it, leave the room, and follow the path back through the basement. You'll emerge in the Royal Bunker, which, fortuitously, is right where you want to be.
Occasionally in ZombiU - and particularly in Ron Freedman's flat - you'll come across Animal Meat. You can also get this meat by killing animals and stuffing their corpses into your inventory. These technically count as food items, but they're just as likely to hurt your survivor as they are to restore any health. You are far better off ignoring these choice menu items and devouring something that's been packaged instead.