On a PC, I've downloaded a list into excel and saved it as a MS-DOS CSV file. When I go to Civi to import it, Civi says it's not saved as a CSV file.
I've followed the same steps I have for a dozen other contact lists.
What am I missing?
I would suggest that you take a look at the documentation: Importing Data into CiviCRM
Read through this and follow the steps carefully. Try to export a small number of contacts - i.e. the first ten or so. If you are able to import these then the problem may be with some of the data in the later records.
You may need to change the export settings so that fields are separated by a semicolon (;) rather than a comma (,).
You may find the answers to this related post helpful: CSV file import problem with file from Mac MS Office
If you are still having trouble it would be helpful if you could paste an example of the CSV file to your post with dummy data replacing any real data. Again, just the first few lines would be sufficient.