The yellow flashes could be a few things but it’s hard to know without the specific pattern and any accompanying beep codes. See here: http://ardupilot.org/copter/docs/common-leds-pixhawk.html
I’m going to assume that it has failed pre-arm check. Have you got it connected to Mission Planner when you boot? If so, are you seeing any errors on the HUD?
Also, and I am sure you’ve tried it, have to turned it on when outside with a clear view of the sky?