Fader mask

I have this stupid n00b question and i’m angry because i don’t know this though i’m working with flash for 4-5 years now!!! Lol!

What the hell do i have to do to mask something that the ends of the mask fade the masked layer?

I tried possibly everything. Except the right thing! Lol! I’m sure i have to use a gradient instead of a solid color. But which one?