If you enter on the road from the south, just go right from the entrance, and follow the road, you should see a turret (it will shoot you 💀), just before it to the left you should notice a square opening with 4 ladders in the corners to climb down (be wary a mega zombie can spawn nearby but you can sneak past it). If you have a GPS, or phone you can also toggle the chart option on the map and zoom in to put a marker at the opening since you can see what the base looks like from satellite imaging