Help me please :)

You can't anymore. It was a one-time only event in February. You can, however, make yourself look like a zombie with items like

Hmm... so nobody can get into the Zombie Den?

People who are still zombies can get into the den.

Except those who stayed zombies. For us ordinary mortals, nope, no Den for us.

Darn, what's in this den?

I can get into the zombie den ;D

edit: there are lots of items, books, minions, wardrobe items, ect.