I made a subsite, it worked beautifully. I want to map

I made a subsite, it worked beautifully. I want to map a URL to it.

The DNS has been directed to my host, hostgator (and that is also the registrar). It has propagated.

I created the site as an ADDON site on hostgator (I was told to do this). I installed Domain Mapping, following all the instructions. The wildcard domain is activated at hostgator, where the multisite is.

The moment I map the domain newsite.com to mynewsite.multisite.com, the site breaks (giving 500 error) and I have to delete it and start over. At least it's repeatable.

Any idea what is wrong? Thanks,
Cindy

PS - I guess there's something major I don't understand. The other subsites I have added do not have an ADDON, and do not appear anywhere in my FTP folders. Where are they? Maybe I was not to create an ADDON? Is that the problem?