While still holding the display panel up, use two fingers to firmly push down on the power supply cable connector from its socket.
Be very careful not to touch the exposed face of the power supply to avoid electric shock from the many large capacitors attached to the other side of the board.
You need to get to 10.6.8 via physical media, then app store update from there. I have read that Apple will send you a dvd for no cost, but the official price is $19.