UPDATE:
I figured it out.
For anyone it might be useful to... my first steps were correct. But there was an additional step involved. I had to go into cPanel for the main hosting account, then go to "Add-on Domains" and then add the new custom domain I want to use (no http or anything), then it will populate the other fields... in the path field it will have the path to the root folder... remove everything except "public_html" - this will make it so the domain will point right to the root of the site. Do this for any each domain you want to map.
Then after this is done, check the domain to make sure it works. Once it does, THEN go to the plugin and configure accordingly.
Now that I know what to do it's relatively easy... but wasn't easy to figure out the first time!
Thanks
Doug