You must import it into the same version of Spine it was exported from. You can run Spine like Spine -u 3.6.33 or enter 3.6.33 in the settings dialog and restart Spine. Then you can import and save a .spine project file. Unlike binary or JSON dat files, you can open the project file with any newer version of Spine. I highly suggest to keep your project files so you don't need to import your data again.
Here is a project file for your 300102.skel file:
http://n4te.com/x/8154-300102.spine
Note it had some warnings when importing because the data was exported without Nonessential checked. This means Spine had to make some guesses, especially for meshes. You should check to make sure everything is OK. You can do the import yourself to see the warnings.
