That video is a little outdated. (But still really helpful! I used it too.)
You no longer need rename .json files to .json.txt. Unity recognizes .json as TextAssets now. (Also, you can use an importer to handle renaming .atlas files. Mine is here if you like.)
Make sure your texture is set to "Texture" under Texture Type, and not something else. (It defaults to "Sprite" if you're working in the new 2d mode.)
After assigning your texture to your material, choose the Spine/Skeleton rather than Transparent/Diffuse shader. If you're using something else, you might need to add a light to your scene for it to show up.
Give those a shot and see if it helps.
Edit: Updated Importer URL.