URL setting

I am trying to have an outside domain name point to a sub domain of my wpmu.
My wpmu is onlineaor.com
The sub domain is askjoe.onlineaor.com
The user for ask joe wants to direct charlestonbuyersagent.com to askjoe.onlineaor.com
He got ccharlestonbuyersagent.com from Register.com
I am using dreamhost for the wpmu

Dreamhost says I need to check the url settings for my application

where do I go to do that and is there an easier way to do this?