The nameservers for your domain do point to dreamhost.
Tracing where the traffic goes, it seems to go in dreamhost front door, but reverse DNS on the IP points to g7s.cn Pulling up that domain, leads to a strange page that I don't understand it's meaning.
in post#8 of this thread you said:
which leads me to believe that another dreamhost account owner, which you are reffering to as "the guy who own the server" already has this domain added on HIS dreamhost account.
Since the registar for your domain is godaddy, I'm not sure what dreamhost's policy is on establishing ownership. The fact remains however, that a domain can only exist on ONE dreamhost account.
Normally when that error is seen it's caused because someone in the past owned the domain, hosted it on dreamhost, then let it go to expiration and later on someone else came along and registered the domain and then tried to host it on their account. In THAT case, the new owner would open a ticket and let support know what's going on, and support would delete it from the other account. Then it could be added to the new account.
In your case, it seems a 3rd party is doing the hosting of the main domain for you, and you are trying to add sub-domains on your account. That won't work. A domain and it's subdomains must all exist on the SAME dreamhost account.
Asking support to remove it from his account will result in the site thats already there being deleted. ALSO, in this case, I'm not sure how dreamhost support determines the owner since it's a live domain and dreamhost is not the registar. You would need to open a ticket and ask them.