you have to activate it like bombs. Choose crossbow first and then use it. At least that´s how it works with the controller so i guess on keyboard it might be similar.

TAB to select it first, middle mouse button to shoot. Default keys.

As above... I think it was also in the tutorial when you get explained how bombs function, or at least when you get your first crossbow during "The beast of White Orchard" quest.

Activate it and then right bumper to shoot (you can aim first or have it just go for the nearest target).

Thanks, I had it equipped just needed the trigger key. I don't remember it being in the tutorial but it's quite possible that instructions flashed on the screen when I was given the crossbow. however, I was a little keyed up at the time considering what I was about to face. Damn silly time to give the player a new weapon to get accustomed to in my opinion. Thanks for taking the time to reply everyone.