Domain Mapping or Multi-domain, not sure if either will do the trick

Ok, I thought what I want to do is pretty easy, but I keep running into road blocks when I try either the multi-domain tool or the domain mapping tool. In a nutshell, I have a custom theme I’ve developed. I want to allow users to:

1. Be able to sign up at one website I own as a subscription service (done)

2. I will then manually create a domain and user for them on the MU site (like http://www.sitea.com)

3. They will then modify their dns / cname or other setting to point to the MU site

4. They will then access the site and modify (through http://www.sitea.com/wp-admin/)

When I tried with the multi-domain plug-in, I could create a new domain (like http://www.sitea.com) in the plug-in, but when I tried to create a new site as the network admin, it wouldn’t just let me choose the site – I had to add a folder below it (I created a subfolder MU install)

Is the domain-mapping plug-in the plug-in that needs to be used instead?

Thanks,

Andrew