So I just got the game and installed it. Then I use the icon on my desktop.
It tells me to enter my key I do that it says checking for updates and then it closes.
When I start again still says enter key. I do it again and nothing happens.
So how can I register my game? And start it?

I had the exact same thing when i first bought the game (retail version). The thing that fixed it was -> (see below)

Seems like

a) the servers are down (again)
b) they are working on the servers to release the patch

In either case, it sucks that you will have to wait if it's a server issue

OR (my case)

c) you entered your activation code incorrectly

Make sure to put "-" between your activation code numbers (like it is written on back of the manual) , i didn't do that the first 5 times i tried and it refused to register, the launcher also suddendly stopped. Also watch out for differnces between 0 en O.

To actually play, I'm pretty sure all you have to do is bypass the securerom, which are...securerom servers, not witcher 2 servers.

Registering the game is just to download dlc.

So, in short, Click "Launch Game".

That's assuming you're not using the GoG version, which doesn't even have to worry about securerom and can just click "Launch Game".

ie: I don't understand what registration has to do with actually playing the game.

Well before i entered the serial correctly and it registered correctly i tried using a crack... which resulted in breaking the game launcher and a reinstall of the game.

It's your choice really

I need to enter my key which I did but servers are down or some other issue. So it closes the launcher without doing anything.
Then I start the launcher again and I need to hack in my key again. But it closes again.
I could be doing it for hours I bet.