My pc is dual boot windows 7 and openSUSE 12.3 (Linux)

I bought this game, through steam, together with roe.
Played it on win 7, and it worked.

Since steam came to Linux, I am trying to reboot into windows as little as possible.
Because Doom 3 should work on Linux too, I used google, and found this: ... linux.html

It all worked fine, but when I start doom 3, it wants me to fill in the cd key.

So I quitted doom 3, en used my file browser to find the cd key in steam on windows.
I found it , it was 16 characters long.

I tried to type it in a few times, but I get invalid cd every time.
Then tried copying the key file from windows to Linux.

That did not work either.

Does anybody know any legal solutions on how to solve this ?

Some one sugested,
find a copy of the D3 GPL code w/o CD key, compile and use that.
I have never compiled anything.
So I need a little help.

Or are there maybe better solutions?

I did found this, but I am already lost on what to download.