While in the main menu with the cursor on "Balls", hit (at the same time): 
BAL   
Toggles in 5, 7, 13 balls (keep entering until you get 13)

1 (top of keyboard) - turn off anti-gravity
2 (top of keyboard) - turn on anti-gravity

While playing some tables, F1 will change the ball type.

(Quoted from a post by Mark Jensen)