Most Spine-Unity classes are in the Spine.Unity
namespace.
Some of the modules belong in the Spine.Unity.Modules
namespace.
The base Spine classes belong in the Spine
namespace.
So for most uses if your class is interacting with Spine classes, you just need to add the following to the top of your script.
using Spine;
using Spine.Unity;
//using Spine.Unity.Modules; // if you are using some of the optional/specialized modules