Day of Defeat Levels
Day of Defeat runs on GoldSource which is one of the orignal versions of Source, a game development engine made by the developers of Half-Life. There are no real offical sources on how to make maps for Day of Defeat other than Holycow's video playlist on it, but if you'd like you can go over the the Day of Defeat forums and see if a experienced player can assist you there.
Need help with making maps? Go to the Day of Defeat store page. Check out the store and forums!
Why Certain Versions of Hammer Don't Work
Newer versions of Hammer don't work because they are for newer games that use Source. Newer versions of Hammer also don't have the same setup required to make goldsource maps. Therefore you must use older versions such as Hammer 3.4 and J.A.C.K. NOTE: Valve did not create J.A.C.K. They only created certain aspects of J.A.C.K. that make the program work.
Is There a video on how to make maps?
So your new to Day of Defeat and its map making aspects. All good, just watch Holycow's Video on it. Video