launcher ex: how to take screenshot in? 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 vol-down+power. But Samsung uses (at least on the Galaxy S2) the combination home+power, where 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.