I have a WP Multi User setup. I have a sub site

Hi,
I have a WP Multi User setup.

I have a sub site rubins.zenblogs.dk for a friend.

This friends own domain name, rubins.dk, should open up the rubins.zenblogs.dk sub site.

I have setup a A record for the rubins.dk DNS which points to the web server, which is a hosted web server.

I have also installed the Domain Mapping plugin so that rubins.zenblogs.dk will be redirected to http://www.rubins.dk.

But going to rubins.zenblogs.dk or http://www.rubins.dk just goes to the hosting providers generic webpage.

Hiw should I set this up so going to http://www.rubins.dk will take me to rubins.zenblogs.dk with http://www.rubins.dk address in the browser?

Thanks.

Regards,
Nikolaj