OK...one more question about this. It is working fine. I can add the Canadian code when I place or edit an ad. If I enter my US zip code, the distances are shown correctly to an ad with a Canadian code.
The remaining problem that I have is that if you are a Canadian and are viewing the ads, the first product in a category shows a field to enter your zip code so that the distances can be shown. That field will still accept only 5 characters so we missed one that needs to be changed. Where would I find that one? A table I assume but which table?