unplug all of your peripherals (everything but the power cord) and then reboot and hold the option key down. Your ML disk should be seen and you should be able to select it and then boot into it.
Read more about this here:
There was nothing to unplug. I am traveling and don't have any extras with me. I eventually force-restarted the machine and it now seems to be installing correctl, though qthe progress bar got to the end, after 34 minutes and then started again with 20 minutes which is where it is now.