3.0 Multisite Domain Mapping – I'm in way over my head.

I’m in way over my head and have no idea what to look for or where to look for it. Here’s the situation. The main site is alarmdev.com. The sub-site is devsite102.com. http://www.alarmdev.com and alarmdev.com seem to work fine. devsite102.com takes me to the registration page for alarmdev.com. devsite102.alarmdev.com takes me to a “site not configured” page at Webfaction. I’ve contacted support at Webfaction, they’ve looked at my configuration there and say it looks fine to them.

I’ve followed the instructions included with the plugin download, but I can’t find any step-by-step installation instructions at WPMU DEV.

Can you give me some pointers or things to check to get to the bottom of my problem?