After Effects cs3 - how to select frame to start effect?

I’m using after effects for the first time for years. I have a simple question and I just can’t seem to figure out how to select which frame I want to start a transition effect to start and end on?

In CS5 you’d have to use keyframes…don’t know if that works on CS3.


TehYoyo is right. Here is some more information about keyframing your effects.