When creating a game, go to "Advanced Options", and select Capture the Dragon Balls from the Game Type drop-down menu. Some maps don't work with the mode, so I'd suggest using esf_cell_day to try it out. Once you have collected a Dragonball (by touching it), select it in your weapons menu, and use it like an attack to drop it. When they are gathered, they glow. Approach them and press 'b' (by default) to summon the dragon. Have fun!