Logic Pro- Getting audio from video to external monitors

I am scoring a film. I have my audio playing through external film. The film that I am scoring has a pre-existing audio track with dialog, and scoring beneath that. The audio of my composed music in Logic is playing out of my monitors just fine. However, the pre-existing audio track for the video will only play from my Mac Book Pro's internal speakers. I went into the video preferences/settings in Logic, and for 'sound output', it is set at 'system sound.' The other options are 'mute' and 'external device' - but it won't let me set it to external device, it's not a highlight-able option for some reason. Any idea what I'm doing wrong?I am using Logic Pro 9.