Pry the square rubber piece in the middle, not the hard plastic square frame.

1) The silver cable doesn’t need any force to insert - just push it gently in and use the retaining flap to lock it in place;

2) The DVD drive power cable is a little tricky because of the orientation - take a look of the pins inside the socket: it is located more toward the outside surface (not in the middle). As such, check your cable orientation before insert, make sure it is in the right orientation such that the holes will meet the pins. It takes me one try to snug it in nicely.