Hate to break the news but you cant equip it. It can only be used if you find the right parts.
Compound crossbow that you scavenge parts for, where do you get the aluminum bolts and the broad head tips they're showing in the picture? By the way a full length arrow like in the picture would snap when fired, and while they would be a relatively short ranged weapon, they would have more power, penetration and knock down than all but a couple of the sniper rifles in the game.

i dont now how i fell about the crossbow, it kinda feels *diffrent* for for bf3