I am a newbie with macs and I have a MBP and I am trying to get a game to open on my external monitor when it is plugged in. Specifically, the game is Neverwinter Nights. This is in OSX and not bootcamp.
You could try making your external display your main monitor. Go to System Preferences>Displays>Arrangement and drag the white bar at the top from the laptop monitor to the external display. Then try opening the game again.