Did you check out the ghost shooter tutorial it will explain a lot of things.
but to do what you want go into the Globalvariables manager and add a new GV called whatever you want.
then make a new event that will trigger when the QB throws the ball. so a button press or something will do (us the mouse and Keyboard object to register button presses.)
now in that event go to System>Set global var to Random(99)+1 (this will return a number between 1 and 100)
now add the function object and create a new function called what ever you want. and in sub events under that function us the system compare global variable to what ever percent you want and then in that action have the go to layout system command.
Now go back up to your trigger and add a call function UNDER the set random num.
**Fake edit**
You could also pass the function the randome(99)+1 and check function.param(1) in the compare condition.