You mean you need 1-in-5 chance to drop a power up?
random(0,5) will generate a random number between 0 and 4.99999999999, it will never be exactly 5. Change your condition to random(5)<1, this will give you a 1-in-5 chance.
Another option: choose(1,2,3,4,5)=5
Thanks for the tip, but I want to also know how to make the events not loop, like:
When I shoot an enemy, it will verify if the variable PowerUp is 5, and when I make that sub-event, it will keep creating PowerUps. Here's an image to clarify: