Click on 'File' then 'Open...' and navigate to your world folder*. Once here, open the folder 'playerdata' and finally the file with a weair name inside it. The window in NBTExplorer should look like the second image attached.
Click on 'File' then 'Open...' and navigate to your world folder*. Once here, open the folder 'playerdata' and finally the file with a weair name inside it. The window in NBTExplorer should look like the second image attached.