Hi,
You should use our latest HTML5 video player - you can create a new one in the Universal Studio (not the old Flash Studio).
Use our DoubleClick plugin (available under the monitization category on the left) to add the ad tag. It supports both Javascript and Flash VPAID playback.
If your ad tag is a doubleClick adTag, put it in the Ad Tag URL field in the DFP Trafficking tab.
If your adTag is not doubleClick, select the VAST Trafficking tab and put the adTag in the Preroll URL field.
If you still can't get it to work - please send us the adTag you are using and we will take a look.
Amir