if your laptop monitor isn't working at all, disconnect it. plug in the external, and then boot. if the laptops lcd is disconnected, the external will be the default monitor. I currently own two laptops with broken screens, they make decent little media centers (not for bluray of course) and sometimes you can get them cheap off ebay.
don't worry if you haven't opened your computer before. you're posting on forums, so you have access to google, and google can show you exactly how to do it. only the video cable (and sometimes a ground wire) need to be unplugged. the hardest part of the process is removing the keyboard (and its not that bad). hope this helps someone, and good luck all.