It depends. If you want a fade in/out at the beginning/ending of an event, then position your cursor at the upper corner of the beginning or ending of the event. The cursor will change and you'll see a little pop-up that says "Gain" with some numbers. Hold down the mouse button and drag right or left and a fade in or fade out curve will appear.
I'm pretty sure the manual covers this much better than what I did, and it has pictures as well.