In case anyone is interested I have solved the problem.

Somehow when I unzipped Afcad I failed to save the tutorial. When I eventually thought of looking it up everything was clear ( fairly).

To add ATC at an airport that does not have it , you need to access AFCAD under "Lists"/ "Comm Frequencies". Then just add a frequency for the Tower, remembering to put the site name in upper case and save the afcad file to your FS9 Additional Scenery.

If FS9 does not already have your site name in it's voicepack it will default to calling the site "Tower" rather than "Site Name Tower" .

However you can add your site name by using Editvoicepack.

I suppose I should have done all this research before asking foolish questions . Dohhhh !