Help With Domain Mapping

I have set up domain mapping on my multisite. I have my multisite set up using subdirectories. I cannot figure out how to get my forwarding domain to show in the url. For instance, my multisite main domain is http://www.designedbydeni.com , I have created a subsite http://www.designedbydeni.com/ddsbartend, I have a domain of http://www.ddsbartend.com that I am forwarding to the subsite. When I type in www,ddsbartend.com it points to to the correct subsite but the domain name shown is the subsite not my ddsbartend.com. How do I configure it where if you enter http://www.ddsbartend.com it points to my multisite but ddsbartend.com still remains in the url, not switching to the subsite url?