Domain Mirror is a plugin to allow one WordPress installation to be accessed form more than one domain.
Download and unpack the archive. If you downloaded the plugin from my site it will unpack to a directory named "AA-DomainMirror", but if you got it from the WordPress plugin repository it will probably unpack to the direcotory "domain-mirror". Either will work, but you may need to rename the latter to "AA-DomainMirror" if you have problems with some other plugins not seeing the changes. See below.
Copy the whole AA-DomainMirror directory to wp-content/plugins/ If possible, don't change the name of the directory, as the AA- at the start is a horrible hack to ensure that WordPress loads this plugin first. This is required as any plugin loaded before it won't be able to see the changes it makes, so will behave as if it's still on the default domain. This puzzled me for quite a while.
Go to your Plugins page and activate Domain Mirror.
Go to the Options -> Domain Mirror page and configure.
For more detailed configuration details see: http://mcaleavy.org/code/domain-mirror/