I have a galaxy s2 and can't take a screenshot.
I've tried taking a screenshot with vol-down+power and power+home. both don't work.
What is the proper combination ?
It has nothing to do with what app you are using – on Android the screenshot key-combination works globally, no matter if you are on your homescreen, in the settings, or in a game.
However, the global key-combination can vary from device to device, depending how the manufacturer implemented it. Like you mentioned: Normally that combination would be
power. But Samsung uses (at least on the Galaxy S2) the combination
home is the big iPhone-like button in the center below your screen.
Press both those keys simultaneously and hold it for about a second.