This SDK is the barebones kit for putting items into Concrete And Steel!
You will also need Unity editor (Free): https://unity3d.com/get-unity/download?ref=personal
You can find out how to import your 3D models from other applications in the Unity Documentation: http://docs.unity3d.com/Manual/HOWTO-importObject.html
Full instructions can be found in "ConcreteAndSteelModdingSDK.pdf" and an example Unity project is in "/Unity/"
For full details, as well as help and bug reports, please visit the forum: http://forum.concreteandsteel.net/index.php?p=/discussion/4/modding-sdk