Throwing knives are equipped to your class in multiplayer in the "Equipment" section. Since they're in the equipment section, you use the same button you would normally use to throw a frag or semtex or whatever, wheny uo hold down the button, a little cross-like aiming reticule comes up. The throwing knife doesn't replace your stabbing/melee knife nor does it replace a perk slot. Lastly, the throwing knife, like the melee knife, is a one-hit kill no matter what. You only get one throwing knife, however you can pick it back up by walking over to it and pressing the action/reload button or by using the Scavenger perk, which allows you to reload your loadout by walking over a dead enemy's body.