You can only win troop instants as event prizes. In the current event they don't exist at all, except where it's offered as a daily exclusive (that happens 3 or 4 times in all). Once you've won them they will appear as inventory items, along with your KP instants and time boosters. You can click on one and it just adds that number of squads to your barracks.
The prizes you mean from this event are just troops, rather than troop instants. Therefore instead of going into your inventory as a useable item, the troops are just added directly to your barracks.