on the overlay bar with the pen tool,
this will add a point (called a keyframe). Add a point
at the beginning and where you will want the dissolve to end.
Then mouse over the first point (or last point if you are dissolving
out of a clip), wait for you pen tool to turn
into crosshairs, click on the point and drag
it all the way down. now you can adjst the timing of the dissolve
by moving the second clip from side to side.
You can erase points by dragging them up and
off the timeline.