As a complete novice, please help! I have several music tracks saved on a USB stick which my car audio does not recognise. The dealer says I need to load a playlist. Can you please guide me, step by step through the procedure as my efforts so far have failed/

To create a playlist:

  1. Load the mp3s in MP3Tag

  2. Create a playlist with File -> Playlist (all files)
    mark some of the mp3s in the list-view panel
    and create a playlist with File -> Playlist (selected files)

But I don't think a playlist will solve your mentioned problem.
What do you mean by "not recognize"?
Are they not played or does the screen only not inform abouth the items played?
Are there tag-informations in the mp3s, that you can see in Mp3tag (Artist, Title, Album ...)?
Are they stored in the root or in a subdirectory?
Do other players play these mp3s?