Like Chuck said, the orange tabs are spring loaded and control the opening for the wire to be pushed into place. My recollection is that you push the orange tabs inward (towards the cabinet in this picture) with your finger or a small flat bladed screwdriver that will fit into the slot on the orange tab and push in. That will open the hole for you to put the wire into the hole.
After the wire is in, release the orange tab and it should slide almost all the way back (see the black wire in the photo) and lock the wire in place. Pull on the wire gently to 
make sure it is seated properly.
