Domain Mapp an external plog to a MU istall

I have multisite installed like this http://www.mysite.com/mu. Now I have an external blog that I am trying to move over to my . How should I go about doing that? Thanks a bunch...

Details:

1-I already have the domain mapping plugin installed.

2- I have a dedicated IP address.

I am stuck here at this point as I cannot seem to get the mapping to work The more detail you provide the better.

Cheers