I need to get domain mapping working. I have a.com I have

I need to get domain mapping working.

I have a.com
I have z.com

I want z.com to pull the site from
a.com/z
but still display z.com to all front end web viewers, and all backend networked together for easy management.

I have domain mapping enabled on both sites.

Right now, visiting z.com/ takes me to a.com/z, and does not display z.com/ to the front end webviewers.

All this is doing is forwarding.

Please assist. I don't know what to do.