To call the dragon in-game, you first must collect all 7 dragonballs to one location and when they start glowing, point at them from closeby and press your USE key (which is b by default). Once you have called the dragon, use the mouse to click on one of the 3 options. Strength gives you more PL, Immortality makes you invincible for a certain period of time, and Friendship increases your wish points in the score (not sure about this) for games played on the basis of which team has more wish points.
[Please note that CTDB (Capture The Dragon Balls) mode is still in it's experimental stages in this mod. It will be perfected after the team has finished working on other aspects of the game. Also, to play in the CTDB mode, the host of the server must have the server running with the CTDB mode option in *Create Game > Advanced*.]