You must first capture every other villain. When you have done so, walk to the main street payphone and you will see it ring. Click on it and now you can fly! Go to the building where Betty Jetty is and fly to the top. Play the mini-game by flying around the projectiles that Betty shoots (I find it hard, but you might not). You will slowly start catching up to Betty Jetty. After a while, you will catch her!