Me and my mates are unable to get to the safehouse provided by the arrows past the police station garage.

I can't find any help on this but everyone I've run this map with has no idea how to progress herer.

The arrows point to doors that completely immovable yet zombies spawn behind them clearly waiting for us.

We've scoured the map and have yet to find any key that would access it.

How do we progress from it? (Third stage, outside of police station garage, right outside the theater).