The dev kit is supposed to be released thursday or friday according to the new community manager. Once it is available, you will be able to get it from Epic Games, the makers of Unreal Engine.

DevKit is what people make the mods in, Anyone who owns Atlas can download it and use it, there is a learning curve to it, if you have never used something like that but, after a bit, I am sure there will be a lot of player made "how to's" out there. Warning on the DL and use, it will be like you are downloading a second Atlas so you need the room on your pc for two sets of the game.. One you play.. and the one that is in the DevKit, program.
(will have all the assets/maps/beasts and well everything that is in the game will be there and you can change the asset or make a new one yourself, even.)
After you are done designing and setting what ever it is you made up.. you then upload it into the Workshop (that you find here on steam) .. again, its a lot of info to learn if you not tried it or have done anything like it before. :) Like me with ark.. I had used blender and all for 3d modeling but, not worked on game assets and the ...well again.. lol its a lot to learn if ya never done but, if you have the passion to create.. maybe you will not mind the learning. ;)