I can't figure this out either. When I wrote the manufacturer, this was the response I got, but I can't figure it out, either:
if you look at the side of your i phone to the charger side, in the corner of the opening crack, use both hands to separate the jacket. Hint best way to remover it with out using force to jacket were it may crack it is to use a coin to the corner to prop it of.
My technique which has been working well for me is to pry the jacket open from the charger side of the phone.
With the phone face down I've put my thumb on one side of the charger opening and my index finger on the other side. I push the thumb up against the jacket's open edge and the index finger down against the other side of the jacket's edge and it pops open.
It's not easy, but at least I know it won't just pop off on it's own!