You have two possibilities.
When you locked the target with your binoculars -->"space". Then you should hear: "Deck gun is ready and manned" and they start to shoot. Then there must be an little yellow deckgun-icon in the left side of your binocular.
Or you climb down the bridge (outside
) and walk to the deckgun and klick on it. The alternative you teleport to the deckgun, also there must be a button on keyboard, but actually i donīt know which!