I don't know if anyone has got dawnguard to work with the creation kit yet as i can not get it to work myself, when i try load Skyrim.esm, Update.esm and Dawnguard.esm everything is fine till it gets stuck on update.esm at 89% but before that there is an error of which is related to dawnguard however normally you would click yes to all on these sort of errors, therefore i think that this error is causing dawnguard.esm not to work.
the following text is the error that appears.
MASTERFILE: LOCALIZATION: Zero entries or empty block size read from file STRINGS/Dawnguard_ENGLISH.STRINGS. Strings will be missing.
I know it is the dawnguard.esm that is making it not work due to the fact i can launch it without the .esm fine, i have tried verifying the files of Skyrim and the creation kit and get the same result.