Buying isn't enough. You have to download and install the addon too. If you are using Galaxy restart it and the download should start by itself. Otherwise search behind the more button for the addon checkbox.
If you don't use Galaxy download the addon. You can find the download on the library page of Witcher 3. The download is about 2,4 GB. After that install it.
Then start the game and the questline of the addon should apeare in the questlog.

But I don't have anything like that in addons, I tried almost everything(screen), and I don't know what to do :/

EDIT.
EVERYTHING is ok now. :)

And the solution was what? (So others with the same problem can help themself by reading your solution.)

Click the game and use MORE > MANAGE > VERIFY / REPAIR function
Now click MORE > CONFIGURE and in the available content select the DLC to install it.

Thanks!