If I'm not mistaken the box retail version of Witcher 3 does not come with a GOG code. If you don't have the game on GOG you won't be able to redeem the expansion key.

EDIT:
Looks like I'm mistaken. The retail version can be redeemed on GOG, so you have to do this first:
http://en.cdprojektred.com/support/gog-com-game-code-boxed-copies-of-the-witcher-3-wild-hunt/

EDIT2:
So after more quick googling, I'm getting mixed results. Do you have Witcher 3 in your GOG library? Did your box version come with a GOG code?

He doesn't have Witcher 3 in his library. He bought boxed version some time ago and I suppose he played it offline. Now he bought online GOG code for expansion pass but he needs to have the base game digital as well. So he is trying to redeem the game (TW3 not the expansion) here on GOG to be able to play it.

And yes, retail version should come with GOG code. I have one lying in front of me.

As for you pabaa, what way are you doing it? Through the redeem page?
It should work that way.
Have you played that game in the past? If so, it should already be registered with GOG to get the free DLC, right? So maybe you already have it tied to an account?
If you tried to go through redeem page, have you done it through GOG Galaxy or browser. In game specific forum there are soem mentions of porblems when redeeming though Galaxy, so try it through browser if you haven't. Also try to read forementioned forum for solution if nothing works, I only skimmed it as I had no problem.
If it still won't work contact support and hopefully they will be able to help you.

I tried redeeming through the web page

I already finished the game in autumn. but offline. I cannot download the free DLCs (the CONFIGURE and EXTRAS button in MORE is grey and inactive). When I click on CONNECT, it asks me for a code - when I input the one from muve.cz, it says the same line again.

Also the Galaxy wouldn't let me purchase expansion pass in the galaxy shop (not available in my region, which is bs) or Hearts of stone (no reason given), which is why I went to muve.cz

That is exactly what the others are talking about. You need the base game on gog for the expansion. And normally in the boxed version of Witcher 3 there should have been a code you can redeem here on the website so that you have the base game here and can download the free DLCs etc.

Oh man, that's it!

Thank you so much, I completely forgot about that code! it's working alright now, may Melitelé bless you!