domain mapping not resolving

I've set up the plugin as per the instructions. But it's still not working in Wordpress Mulitsite. Here's what's happening:

When I browse the domain, I get the message that it can't be found. It looks something like this:

My main domain, on Rackspace Cloudsites, which houses the WP Multisite install is; http://www. domain.com. The domain, set up as an alias in Rackspace that should be working is http://myuniquedomain.com, and it points to the http://www.domain.com/unique.

When I browse to http://myuniquedomain.com, I get the message that Safari can't find the server at myuniquedomain.com.