Steam versions of the game are already patched with the Gamer's Ed of Redkit, so you don't need to dl anything but the mods themselves.
Follow the instructions which follow "Don't feel like creating a mod"? on this page : http://redkit.cdprojektred.com/index.php?c=download SKIP step 1, you already have that patch.
If you're a modder, you'll need to make an account on the Redkit site (you can use the same username/password as this site), and download the modders version.
But in order to use the editor, you'll need to redeem your GOG backup of TW2, which you do here: http://www.gog.com/witcher/backup
Make a GOG account (you can use the same stuff as here again), and enter your product code; the game should show up in your library.
Hope this helps.