For using the Creation Kit with Dragonborn or other DLC from Skyrim game, there is an error that occurs on loading the CK. This is how to fix it:
You need to make the following changes to the SkyrimEditor.ini file, located in C:\...\Steam\SteamApps\common\skyrim as follows:
Add these to the end of the following section:
Copy and Paste the following into the [Archive] section:
SResourceArchiveList2=Skyrim - Shaders.bsa, Update.bsa, Dawnguard.bsa, Hearthfires.bsa, Dragonborn.bsa
After the following changes have been made, and you save the file, you will be able to edit the masterfiles and any mod that uses any of the DLC.