Hi, You need to open the buckle by depressing both buttons, then turn the buckle 90*, with buckle open, slide the entire buckle forward, positioning the fin post into the larger hole on the buckle. This will allow the buckle to come off completely. Note: the post has a ridge on it, and will only allow the buckle to come off in one position, so for your initial 90* turn, if it does not work, turn it 90* the other way. It will also be fairly stiff (by design), requiring a bit of energy. You may want to wear some light weight gloves the first time you do it. Just a precaution.
I hope you get it figured out.
Best,
Just got a pair recently and had the same issue. "A bit of energy" sure is right - thought I'd break a finger! Finally did get the straps off in the way suggested. Haven't tried the fins yet (going on a trip later this month), but they came very highly recommended by my LDS.
thanks! i'll give it a try in a bit