First, you will need to beat the easter egg once to be able to play as "Elvira". Then you need to beat the easter egg again, while playing as Elvira. For the easter egg guide, please see this guide at the achievement guide for the Soul-less achievement.
The code for playing as Elvira is
Down Right Down Left Up Right. You do need to have the correct map (Attack of the radio thing) selected. You can quickly type in the code when joining mid game. Keep in mind that typing in this code slowly does not work.