Domain Mapping 404 Error

I've read the instructions as well as a few forum posts but I still cannot get any clear directions. I am new to Domain Mapping plugin first up. Per Aaron I need to have a dedicated ip address to make this plugin work. I did just that so now I have a dedicated ip address for my wp site.

Domain Mapping plugin installed and configured successfully.

Domain mapping Configuration Server IP address exactly correct and also tested in the browser and it works.

For the blog in question I head over to the registrar at netfirms.com and entered in Domain Manager A (address) which is the same ip address as the one in Super Admin/Options Configuration Server IP address in Domain Mapping.

Now I go to admin for that blog under Tools/Domain Mapping and enter the blog address in Custom Domain field and I get error

Not Found
The requested URL /blogname/wp-admin/tools.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I also tried the blog's original website address in the Custom Domain field but get the same error.

So I head back over to Super Admin/Options and try the different Administration Mapping Options but keep getting the same error.

When I manually enter the site address site.com in a browser it only takes me back to my main wp site.

btw even if I leave the Custom Domain field blank I still get the same error.

Again I am new to the plugin and just trying what I think should work. Help or documentation will be appreciated.

Also if netfirms.com is not suited for the purpose can I get a list of registrars apart from just godaddy that will work with this plugin? That way I can let my users know this rather than trying to experiment.