(I already downloaded the game but i cannot open it)

More details will be needed to help you. Please describe what's going on and especially where things start going wrong.

I have noticed that simply clicking on the launch install button doesn't always work correctly.

I suggest right-clicking then selecting launch installer from the menu that arises there.

I've had to do the same thing as notredatall before. Also, if you are using the old downloader I'd suggest using the new one. I've only downloaded one game with the new one but I didn't have to right click to install. Good luck.