Domain mapping for primary site

I'm likely going to feel like a complete moron for posting this, but it's been three days and I can't, for the life of me, figure out what to do.

I've replaced WordPress MU Domain Mapping with WPMU's Multi-Domains and Domain Mapping plugins. (They're more sleek and look much nicer -- and way more user friendly.)

My only issue is that I can't seem to figure out how to map a new domain to the primary site. As it stands, we own slantpoint.com and slantpoint.co. I want to override slantpoint.co, which has been the primary domain up until this point. I tried doing it in the backend, to no avail, and ended up screwing up a lot in the process.

Hope someone can be of assistance.