I had same problem and answers above didn't help me. It turns out that new macbook pro's doesn't have power on pads anymore.
The solution is:
'''Shorting Pin #5 on the keyboard connection (#5 from the right with battery facing up) of the logic board to a ground point will boot the laptop.
This is true for most late model macbooks and macbook pros from 2008~present.
Some models have different ways to do this, however this will work for 90% of the newer unibody laptops.'''
Found it from here: [http://www.logisticsct.com/blog/tag/macbook-short-pins-to-boot/|source of solution]