In multi-player mode, join or make (recommended) a small level. Bunker is good and be a predator with a disc when you spawn. Throw the disc somewhere you can quickly get too and press F1 to change to the no disc predator, 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 acquired it.