Flash Video Overlay Ad Help!

I am trying to figure out how to create a video overlay for MTV. On the ad spec site, they provide specs and templates for the video overlay that shows up on the lower third of video and expands to a full all. You can see at
http://adspecs.mtvn.com/site/spec/interactivevideoproducts/lower3rdinteractivead.html

For the ad on the lower third of the video (this is the semi transparent banner ad on top of the video), the specs indicate that the “opacity of creative’s background should not exceed 70%. Text and images may be 100% opaque.”

I am not sure how to make the background semi-transparent. Please help!