1. Complete the easter egg up to purchasing the escape door.
2: Gain rainbow perkaholic or at least buy staminup and the double jump perk.
3. (MOST IMPORTANT) You need to have the "mystery" powerup, it dissolves the invisible wall an allows entry, get it from killing zombies, it looks like 2 crossed swords or knifes.
4: Once you have the mystery powerup, activate the fling trap that is used in the 4th step of the main easter egg FROM THE CHURCH SIDE. You will not get enough height if you do it from the bar/cars side. Run to the trap while being aligned with the window, hit it, and double jump in midair when you are a little past your peak fling in order to hit the entrance. You may have to crouch to squeeze in, but once you are in zombies cannot attack you but you can attack them. You will find raygun 2.4 and assault rifle. ENJOY!