I was browsing the Dragonborn.bsa file (with a 3rd party bsa browser) and I only see the PEX files. Where are the source files again?

I feel like this is a dumb question but I am confused as I have only obtained the Skyrim source files (have not tried to obtain the source files for any of the DLC untill now) so it has been a while.

Just in Data/Source/Dragonborn. You have to move them to the Source folder, in with the rest, then you can use them in the CK.