All credit for this guide goes to T0b3 over at X360A. I'm just passing it on to TA.
Chapter 1 - Part 2 - The Zombie Reloaded:
Found near the exit of the stage (Not the goal, follow the road to the barricade. From there on, go right and enter the fenced area).
Chapter 1 - Part 3 - Finding Zombies:
He spawns by the barricade where you need to go to finish the mission when you've done all the objectives.
Chapter 2 - Part 1 - The Zombie Brothers:
Go straight, it's to the left of the white building that looks like a factory (the bus station, if you see it from the other side).
Chapter 2 - Part 3 - The Da Zombie Code:
If you follow the road right until you reach the purple car and then take left, he should attack you when you are following the road up to the barricade.
Chapter 3 - Part 1 - How I Met Your Zombie:
Turn left at the beginning then keep going straight forward over the lawn until you reach a place with two tents and two fireplaces next to two houses. He will spawn in that area. You might have to go a little bit forward for him to spawn.
Chapter 3 - Part 3 - My Fair Zombie:
Follow the road until you reach the red truck then take left and run over the lawn, past the gate on the wooden fence. He will be in this area, also where you go to finish the mission.
Chapter 4 - Part 3 - Zombies N The Hood:
Two of them on this part:
Zombie 1 : In the beginning, when you turn left he should be there in the lawn area. Check the road because sometimes he spawns there too.
Zombie 2 : In the area between two barricades. A entrance by the truck - its the same area you go after finishing all the objectives.
Chapter 5 - Part 2 - The Zombie After Tomorrow:
In the big parking area with Meter-maid and Franken.
Chapter 5 - Part 3 - Gone With The Zombie:
He will be running towards you, from the fenced area you'll see when you start the mission.
All the zombies disappear if you finish all the missions, so make sure you get the CodeBase zombies before you finish the level.
(Note: you can either wrangle or Range attack kill the Frozen Codebase zombies, they both count towards achievement).