1. File > New Account
2. Select "ZoneEdit.com Dynamic DNS Service" from the Service popup menu, and click on the right arrow. (If you don't see this as an option, then make sure that the file "MacDynDNS.xml" is in the location ":System Folder:Preferences:Dynamic DNS Prefs f:Services:" and that it's a preferece file)
3. Enter "yourdomain.com" (no www) in the hostname field, and your ZoneEdit username and password in the UserID and Password fields respectively.
4. In the next screen, choose "Use Local Address" unless you're using an internal network via NAT. Click the right arrow.
5. You should probably leave this next screen blank unless you're also hosting your mail server and the ip for that is changing as well. Also, wildcard mapping isn't supported very well, so don't check it.

If you're using a NAT router, be sure that it has port redirection turned on for port 80 to point to your machine's internal IP.

That should be it. If you continue to have problems, check your TCP/IP settings (Apple Menu > Control Panels > TCP/IP) to make sure you even have an IP address.