Domain Mapping - Mapped Domain Issue

I am having some problems getting the domain mapping plugin working.

My Wordpress Multi User install has a dedicated IP and when I visit the IP in my browser I am taken to my wordpress sign up page. I believe the set up is correct - atleast that part anyway.

As I understand it, in order to map a domain to one of the blogs I need to add "A Name" record containing the IP of my MU install. I have done this but it has not worked.

Please could anyone offer some advice on how to get this working?

Thanks in Advance