Do you have transparency now(click the checkerboard under the comp window)?
If you do then you need to render out file that supports transparency, like QT_Animation & specify that you want an alpha channel to be exported. For a QT, send your file to the render queue, and change the click on “Lossless”, select Quicktime(change the codec to animation) and change your video output > Channels to RGB + Alpha.
Johnny Cuevas, Editor
Thinkck.com
“I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.”
—THOMAS EDISON on inventing the light bulb.