I've tried pushing against the back as well, but either I'm not doing it well or it's not really helping.
Oddly enough, today I decided to give it another go, and I managed to pop it open. It went once, and I closed it again...and popped it back open again. Took the film out, and decided to close it, thinking since I got it twice in a row I would get it again, and maybe I could get it going properly. Well, back to not being able to get it open. Haha. I'm going to use the film as my first test for home development, but as noted in this thread by others...I'm guessing I probably didn't have it spooled on properly, so likely nothing on there.
Was very odd how all of a sudden it worked as it's supposed to, and just as quickly decided not to again. Oh well.
On a side note, the ebay seller I bought the FM from just finally sent it yesterday. 6 days after ending. Lame.