This is how you obtain the mode "Nazi Zombies" in the fastest way possible.
Step 1 : Go to Solo Campaign
Step 2 : Set your difficulty level to easy.
Step 3 : Run your butt off towards the Stars on your hud.
Step 4 : Beat the game in about 5 hours or so.
Step 5: Wait through the credits which are about 10 mins long.
Step 6 : Play Nazi Zombies with up to 3 other players.
Hints for Nazi Zombie gameplay :
1 : NEVER PLAY ALONE. Or at night. (I'm scared of Zombies)
2 : Use your pistol for the first wave.
3 : Buy the M1 Garand ASAP!
4 : Repair ALL barriers ASAP!
5 : DONT turn your back on zombies, they are faster than you think.
6 : Open up the door after you earn $2500. The door costs $1000 and the BAR on the other side costs $1500.
7 : Kill zombies!!!!