When you start up with WP.MU you need to bring your own domain with its name servers configured to point to:
ns13.psekhosting.com
ns14.psekhosting.com
This is a quick guide to what name servers are and how you can configure them with different domain hosts.
Please note that your domain host will, almost certainly, also have help and information about how you can configure your name servers… and once you’ve done it once, it’s easy 
So what is a nameserver?
Name servers are essentially the configuration parts of your domain that point to the server where your website sits. For example, when you buy a domain it might point to ns1.godaddy.com and ns2.godaddy.com – this means its pointing to the godaddy servers. Here’s Yahoos explanation too.
So why do I need to change them?
Well, your new WordPress MU install will be sitting on pSek servers, and so you want your domain to point to them… if it doesn’t, then your install won’t work, so it’s really important that you change them in good time before you run the install!
I’ve changed them but it’s still not showing up! What’s going down!!!
Well, first you have to get started with us (if you haven’t then you can change the name servers and you’ll still see nothing), but if you’ve already done that and you still can’t see anything then this is most likely for one of two reasons:
a. Your name servers haven’t resolved yet (sometimes it can take up to 24 hours for a change in name servers to be recognized by an ISP so you might have to wait a bit)
b. There’s a mistake in your name servers.. please double check them, make sure that there are no other name servers listed
So, how do I change my name servers?
Depending on your domain host, here’s how you do it:
How to change godaddy name servers (at the bottom, you want ‘Custom’) – Also there’s a video here
How to change Yahoo! name servers
