Sorry if this was asked before, but Im quite new using steam. I got both, Skyrim and Dawnguard, and I wanted to know how to install the CK. Or I have to buy it?
Sorry if this question is just stoopid, but well, I just dont know.

The http://www.creationkit.com/Main_Page is free. To get it, Steam > View > Tools > Creation Kit > Install.

To open Dawnguard, you'll need to edit Skyrim\SkyrimEditor.ini which will be there after the Creation Kit is installed. Search (Ctrl+F) SkyrimEditor.ini for "SResourceArchiveList2" and edit it so it reads:.

SResourceArchiveList2=Skyrim - Shaders.bsa, Update.bsa, Dawnguard.bsa

Thank you very much!! Im installing it right now