How do I make the game respond to the second click?

From ggc

After I change from stage==0 to stage==1, how do I make the game react to the second click? Do I need another:

if (left.intersects(click))

and so on? If not, how else would it be done?

Yes, in your advanceFrame method you'd need

if(stage==1)
{
     //second click responses here
}

We'll talk about ways to shorten and simplify this code in the coming 2 weeks of class, but for now, just add code to get it working.