You need to squeeze the connector top to unlatch, then pull it parallel to the LED backlight board.

My hard drive replacement (a Mach Xtreme Nano PATA ZIF 120GB SSD) actually went in the other way up from the stock Apple Drive. ie The ZIF connector is seen when put back together, rather than under the drive as is the case with the Apple Drive (as seen in Steps 10&11). This also meant the cable came up past the drive on the inside of the bracket then to the mother board. I had tried several times the same way as the Apple drive was, and the cable kept being pulled out of the ZIF connector when putting it back together. Luckily this did not damage my drive.

Rather than through the vent at the back. Ease the bottom of the front bezel away from the screen and then in the gap created between the screen and the front bezel, insert your card here and push up to release the latch. This is especially the method to use, if you have already tried everything else above, as you may have bent the latches, and you'll never get them to relase the conventional way.