Very nice set of close up photos of the hard drive cable IR sensor ZIF connectory are here: https://discussions.apple.com/thread/708...
Also note there are 3 photos in this step, you can view them by hovering over each one. Click to enlarge.
My apologies, I was incorrect in my previous comment. The ZIF connector is a little flap on the hard drive cable that can be opened by flipping it up. This makes extracting the IR sensor easier. Removing the hard drive cable from the bracket may also make removing the IR sensor from the ZIF connector, but only if you've opened the flap. The new cable should already have the flap of the ZIF connector open, so make sure to close it. But does anyone know how you can tell if the IR sensor is connected? It's so small it's hard to tell if it can slide in further.
This section is backwards, do not try to remove the IR sensor ZIF connector until you have removed the hard drive cable gently from the bracket as @Skyynet says above, by using a "fingernail (or the spudger) and lift the side of the socket opposite to the cable" gently, it's glued on. Then you can wiggle the ZIF connector off of the hard drive cable with your fingers.