It opens really easy with the viewfinder cover as a leaver. The small button to push is a "hook" to push aside (towards the lens, not into the body). I can imagine that if you don't know this and try to pull the door open, it will brake off easily. It's made of the kind of plastic that isn't flexible.
My viewfinder cover was still present ;-) and I did have the manual, but if you don't have both, it is likely to go wrong.
Here is a copy of the manual (click to enlarge):