Google sitemap verification: 403 error

I'm sure someone will tell me where I should have looked or that I'm posting in the wrong place and make me feel really stupid but I'm just too tired right now to care. I've finally got this site working after 5 weeks of long days/nights and I'm ready to add the Google sitemap. BUT I can't verify ownership with Google Webmaster Tools with the uploaded html file or verify the Google sitemap. I get this error for both:

Forbidden You don't have permission to access /sitemap.xml on this server. Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

I'm using ZC v3.1.9.f
addon: Simple SEO URLs
site: http://www.stockupfeed.com

I've got 15 other sites (none of them Zen Cart) verified with Google and sitemaps working so I'm sure it's something specific to ZC that I haven't learned yet. Will someone please give me some advice on how to get rid of the 403 error. I would be oh so grateful!

Thanks,
Sharon

What are the permissions set to for /sitemap.xml ?

Thanks for the prompt response. Permissions set to 644.

Try 755 or last resort 777

Tried both 755 and 777. Still 403 error. Do you think it has anything to do with Simple SEO URLs?

Do you think it has anything to do with Simple SEO URLs?
Very well could be and there is a thread specifically for that 3rd party module where you may well find that this has already been addressed versus the General Questions, which is mainly directed at default code issues