In multi-player mode join or make (recommended) a small level. Bunker is 
good. Be a predator which has a disc when you spawn. Throw the disc somewhere
you can quickly get too and press [F1] to change to a no disc predator. There
is only one, which is called Assault. When you spawn again, press the disc 
retrieval key (default is F). The disc should actually come to you. You will
have the disc but for some reason you cannot use it, even though the item 
indicator shows that you picked it up.