The Add Mix button not appearing is usually the first sign that something's wrong, but that is usually accompanied by an error message in the Console panel. Sometimes you need to enable all errors for it to come out.
The mesh not appearing just confirms it. But it's quite possible that nothing went wrong on export and the problem is with the setup in the project.
I'm a bit busy these days, but here's what you typically need to do once you have all the assets:
(0) Make sure the import settings of the PNG are correct. Set the maximum size larger if you have a large atlas. Make sure it's imported as a texture (especially if you project has 2D defaults). Maybe also remove compression if you want things to be clean. Also make sure you don't rename this file (this caused me problems recently. Not sure if it's a bug.)
(1) Create a new Material (asset)
(1.1) drag the png onto that material. <
I accidentally skipped this and it caused me errors before too. it was hilarious.
(1.2) set the material's shader to the Skeleton Shader.
(2) Create a new Spine Atlas (asset)
(2.1) Drag the atlas.txt file onto that Spine Atlas. <
I think the .txt is important. Unity doesn't play nice with unknown file types and you need to let it know that it's a text asset.
(2.2) Drag the Material onto that atlas.
(3) Create a Spine SkeletonData (asset)
(3.1) Drag the JSON onto that SkeletonData
(3.2) Drag the Spine Atlas asset onto that Skeleton Data.
(4) Place a SkeletonAnimation GameObject onto your scene.
(4.1) Drag the SkeletonData asset onto the SkeletonAnimation component.