Same way you equip the bow.

Yeahhh, I try to equip them by moving them onto the crossbow or bow respectively which I have equipped, and the ammo counter still says "zero." Maybe it's just me.

Maybe. I don't know that YOU have to equip arrows

That is exactly what I was thinking. I don't remember equipping them. Seems like if I had them, they could be used. perhaps you have the wrong type of arrows?

as i remember, you dont need to equip arrows. just being on the inventory is needed to fire the bow.

Your character will automatically "load" arrows into a bow. If you move your mouse pointer onto a bow, the descriptions will show how many ammo are left "inside" the bow. When a bow becomes empty, the character will automatically take a bundle of arrows from his inventory (if there is any,) and "load" the arrows into the bows. Crossbows and bolts work the same way.

my chaacters vont load any incidary arrow in to a bow/crossbow, do they have to be spesial bows/crossbows?

Yes. In vanilla, only the incendiary bows fire those type of arrows and vice versa. In Age of Redemption mod, you can easily switch ammo for either bow.