You do need to load some or all music files into mp3tag. You may want to use the filter function to limit the visible selection. You can select one file, several files (use Ctrl or Shift + Click), or even all files first. Then choose the Action(Quick) tool and use the suggested parameters I provided. Then click OK to execute this action.
If you don't select at least one file, the Actions are greyed out and cannot be used.
If this doesn't get you going, please post some examples with actual data and screenshots so a more accurate solution can be provided.