There is no such thing as "choose event", but there is choose() expression. You can use it to create conditions like this:
> Every 1 second
System compare two values choose(0,1)=1 : Create enemy
So every second there will be a 50% chance to spawn a new enemy.
What i want you to answer is to how you write a choose expression. On compare instance variable you can write choose("Value", "Value3", "Value4"). The way i write it doesn,t work.