First off, I highly recommend using the XPression Video Coder to transcode your quicktime files to a native XPression codec. They will play much better.
To get the video to play there are a couple different approaches (depending on the end goal). If the video is a looping background, then normally in the material you would click Free Running and Auto Start. This will start the video playing whenever the scene is taken on air.
Otherwise, If the video is more for an effect, like the in/out animation for a lower third; you can drag the material onto a track on the scene director. Then when you put the scene on air and play the scene director (usually from the Sequencer) then the video will play in sync with any other animations you have in the scene director.
#XPression