My newly mapped domains only show up as 403 error pages!

Hi, I just purchased and installed the Domain Mapping Plugin. I mapped three different domains to three sites in my multisite WP installation (subdirectories). BUT when I try to navigate to the sites using my browser, I just get 403 errors!

-- All my domains are pointing at my hosting package which is called [PROVIDER]HOSTING SHARED.

-- I have not messed with the DNS at all. Do I need to do this in order for the plugin to work?

-- Here is the error message:

403: Forbidden

This error message is generated when the web server is trying to access a file that does not exist or has been configured incorrectly

Troubleshooting suggestions:

Ensure that you have a valid home page defined in your website directory (example: /htdocs/index.html, /htdocs/index.php). On Unix, this is case sensitive and must be all lower case.

In your Account Manager, under Hosting Tools, click to .Reset File Permissions..

Please help!