All you need to do is input a code after witnessing the ending cutscene at least once. If you have the Soul Key achievement / trophy, you can play as Elvira.
#1. Complete the main Easter egg.
#2. Load up Zombies > Custom Game.
Then pick Choose Film > Attack of the radioactive thing.
#3. Before entering the game, input this code:
Down, Right, Down, Left, Up, RightIf you input the code correctly, the menu will change and show Elvira's picture. You'll need to input this code every time you want to play as Elvira.
If you don't see Elvira's picture, you were probably too slow to input the code. Go back and enter a custom game again, then quickly input the code.
Cheat Sheet for all maps codes:
More here:
http://www.escapistmagazine.com/articles/view/video-games/wa...