domain mapping issue – not working?

So I turned my maindomain.com into multisite, I chose directories instead of sub-domains. I followed the instructions. Let’s call second domain seconddomain.com

So I created maindomain.com/seconddomain

I added seconddomain.com, instead of showing up, it shows…you know that default page most webhosts have before you add your own index.html/.htm/.php/etc…? THAT is showing up for seconddomain.

I think I missed a step

maindomain.com/seconddomain & seconddomain.com are both showing the same default page.

Should seconddomain.com should be a parked domain, and add-on domain or anything else on my cpanel?

PLEASE HELP. This is the first time I try Multisite