Looking flat into the opening you should see a metal part far in.

With a flat driver try to get between the plastic casing and this metal part to pry it out of the locking notch.

Image 3 illustrates the metal tab you need to press down for the side to open.

One more time: Make sure you are getting between the casing and the metal tab. Inserting and twisting a small screw driver worked for me after a few tries.